Q: I’ve heard I should prune trees and shrubs when I plant them. Is this true?
A: Pruning heavily at planting time is no longer recommended. There are two exceptions: bare-root plants and plants you dig yourself to move to another area. Removing crossing and damaged branches will allow more energy into making roots and not leaves, which will reduce the water requirements for transpiration during the first season. Be sure to monitor watering needs if soil becomes dry.
Q: I have two different crabapples in my yard. One is beautiful and remains healthy throughout the summer, while the second tree develops spots on the leaves which eventually fall off. Is there something wrong?
A: The leaf disease called apple scab is caused by a fungus. Cool, moist growing seasons favor this disease. Crabapple varieties differ in their resistance to this disease. Fungal spores infect the leaves in early spring, just as the buds are opening. As the leaves expand, they become spotted and ‘scabby’. Severe infections can cause the leaves to drop prematurely (and may also infect fruit and twigs). A couple of years of defoliation will not hurt an otherwise healthy tree but repetitive attacks year after years can weaken the tree and leave it vulnerable to insects and other diseases and eventual death.
Management of the disease includes cleaning up infected leaves that have fallen to the ground. The fungus can over winter in these leaves which infect new leaves in spring. Fungicide sprays are available, but must be applied in spring as new leaves emerge, and repeated at labeled intervals. There are a number of excellent varieties of crabapples resistant to apple scab.
Q: We have a small tree, about eight feet high that was planted in the wrong place three years ago. We would like to move it to a better location. How do I dig it up so that it survives the move?
A: Before moving any tree it is important to consider that any tree taller than 8 feet, or with a trunk larger than 2 inches in diameter, is difficult to move without special equipment. If a plant is movable, the best time to move it to a new location is in early spring, while the plant is dormant. Moving a plant while it is actively growing will diminish its chance of survival. This is particularly true of conifers, which have only one growth spurt a year.
Moving a tree to a new location will require trenching, digging, and severing its roots. Several days before digging, water the plant to ensure moist soil. Moist soil will hold together in a ball and adhere to roots better than dry soil.
Q: I planted a lilac bush about five years ago and so far it has only had a few flowers. How can I make it bloom?
A: This is a very common question with many plants. There may be numerous reasons why your plant will not flower. First, age may be a factor. Many woody plants go through a vegetative phase of growth before it produces flowers. This juvenile stage can last anywhere from 2-10 years, depending upon the species. Crabapple, wisteria, and lilac are some of the more common plants that can go through this phase.
Temperature can have an effect on flowering plants. Sometimes a plant may survive the winter only to have its flower buds damaged by a late-spring frost. Inadequate snow cover may not provide the insulation some plants need causing damage to flower buds.
Pruning at the wrong time of year can eliminate spring flowering. Most spring-flowering shrubs form their flower buds in late-summer and early fall. Pruning anytime during this period will remove the flower buds. The best time to prune spring-flowering shrubs is just after they finish blooming. Also, as shrubs age, older stems should be removed to allow younger stems to grow, which results in a more vigorous and better flowering shrub.
Finally, consider the growing conditions for your particular plant. Plants in need of full sun will flower poorly if grown in the shade. Fertilization, especially with excess nitrogen, will promote vegetative growth instead of flowers.
Q: I have tried to grow Eastern redbuds in my yard. Each one died after a relatively cold winter, and I desperately want to grow one. Any idea what I am doing wrong?
A: You might not be doing anything wrong. Eastern redbud, Cercis canadensis, is a 20 to 25 feet high native tree that occurs over an extended range of the Eastern United States. Plants that are grown from a southern genetic source are not as cold hardy as a plant that has been grown in northern areas. It is recommended to purchase redbud trees originating from a northern source. Eastern redbud grows best in a moist, well-drained soil in part-shade or full sun with adequate moisture. Plants do not tolerate standing water. They will benefit greatly with a 3-4 inch layer of organic woodchip mulch.
Q: Can you suggest a few shrubs that will work in a dry, shady area?
A: Plants that will grow in dry shade include red chokeberry (Aronia arbutifolia), common sweetshrub (Calycanthus florida), gray dogwood (Cornus racemosa), American hazelnut (Corylus americana), St. Johnswort (Hypericum), Virginia sweetspire (Itea virginica), spicebush (Lindera benzoin), bayberry (Myrica pensylvanica), nine bark (Physocarpus opulifolius), jetbead (Rhodotypos scandens), wayfaringtree viburnum (Viburnum lantana), and blackhaw viburnum (Viburnum prunifolium). All plants should be monitored for watering needs in dry periods and will benefit with a layer of organic mulch.
Q: Is there a vine that will attach to my brick wall without support?
A: Yes, there are several clinging vines that will attach themselves directly to a surface by means of holdfasts (adhesive discs) or small aerial roots. They work best on a rough surface such as stone, wood, masonry walls and tree trunks. Trumpet vine (Campsis radicans), climbing hydrangea (Hydrangea anomala ssp. petiolaris), bigleaf wintercreeper (Euonymus fortunei), English ivy (Hedera helix), Virginia creeper (Parthenocissus quinquefolia), Boston ivy (Parthenocissus tricuspidata) will all attach to a brick wall.
Q: How can I attract more butterflies to my garden? Can you recommend some plants?
A: Select a sunny spot, protected from wind. The more sunlight, the longer butterflies will stay to find food, shelter, and mate. Some shade is important for resting in the heat of the day. Choose a mixture of perennials and shrubs to provide a continuous source of nectar for your butterflies. Suggested shrubs include: butterfly bush, blue spirea, cotoneaster, deutzia, hazelnut, New Jersey tea, St. Johnswort, sumac, and weigela. Suggested perennials include: bee balm, butterfly weed catmint, chives, coneflower, daisy, daylily, gayfeather, Joe Pye weed, mint, phlox, thistle, verbena, and yarrow.
Q: My maple roots are growing in my lawn making it difficult to mow, can I cut them or cover them soil?
A: Major tree roots often grow within a few inches of the soil surface. Some species, such as maples, grow roots particularly close to the surface. Removing these roots may disrupt the moisture supply to the tree, causing serious stress. Covering them with soil could cut off the oxygen supply to the fine roots in the soil below. Both situations could lead to decline. The best solution is usually to mulch the area under the tree with compost and/or wood chips. These materials are porous enough to allow sufficient oxygen supply to the soil and may actually encourage fine root growth.
Q: We have been having our large ash tree treated for emerald ash borer for 5 years. Do we need to continue treating?
A: Yes if you want to protect your tree. Emerald ash borer populations are still high in some areas and those where populations have declined the beetles are still able to fly to non-infected trees. Trees exhibiting more than 40 percent decline are not likely to recover. Trunk injections with Emamectin benzoate (Tree`Age) provides at least two years worth of EAB control to large trees under high population pressure.
Q: Can I use dormant oil this spring to treat magnolia scale?
A: Dormant oil is a heavy-weight horticultural oil used to smother over-wintering insects. It does not work on all insects; however it will work on soft scale species such as magnolia scale. To be effective it should be applied in early March, when temperatures remain above 40 degrees and more importantly, before leaves start to emerge. (magnolia scale leaflet)
Q: What is the best evergreen to use as an indoor Christmas tree?
A: As you search for that perfect holiday tree, careful selection for quality and freshness can keep the holidays safe and attractive. Needles should be firmly attached and pliable. Pines retain their needles the longest, with Scots and red holding their needles longer than white pine. Spruce trees drop their needles the fastest.
Q: Rabbits are chewing the stems of my shrubs. Is there a way to discourage them and protect my plants?
A: Rabbits can harm plants by eating small twigs and buds or clipping stems at a clean, 45 degree slant or knife-like cut. Rabbits generally feed no more than two feet above the ground or at snow level. The best way to discourage and protect plants against rabbits is to enclose the plant in a cylinder of fencing. Chemical repellents will also discourage browsing but will need to be reapplied after a heavy rainfall.
Q: We had a row of junipers planted in late October but in our opinion they were planted too high and then mounded with mulch up on the trunk. The ground is now frozen, is there something we should do to protect them?
A: If your juniper appears to be planted much too high, that could cause problems in the future. Erosion of the mound of soil could expose roots to the cold of winter and to drying air. Since it is frozen leave it alone, the soil and mulch provide some insulation for winter, then next spring when the ground has thawed, consider having the plants re-planted to the correct depth.
Q: If a white oak has what appears to be some “rot” under the bark on the lower portion of the tree, near the base, should it be opened up and cleaned out or is it best to leave it alone? There are no visible holes in the trunk, but when tapped the trunk seems “hollow”. I have seen different opinions on the internet from what seem to be reliable sources. Some say open it up and clean it out then let it dry, others say to leave it alone. What do you think?
A: If there actually is rot in the trunk it will continue to rot and get worse over time. Should the tree be hollow it could become a hazard and crack or break in the right circumstances. It is not our recommendation to clean out cavities and wounds. Every situation is different. But when a wound has occurred the tree naturally develops a layer of cells that act as a barrier to ‘compartmentalize’ the wound from the rest of the tree. You do not want to compromise this area. Have an arborist look at the tree and evaluate it for the best diagnosis.
Q: We planted an ornamental plum tree 15 years ago. This past spring we noticed some strange, very dark, hard growths on a few of the branches. By the end of the summer they were everywhere on the tree and starting to affect a second tree nearby. What is this and how can we control it?
A: The black growths on your tree are due to a fungus called black knot. This is a serious disease that affects plums and cherries (Prunus species). The conspicuous black gall does not appear until the second year of infection. As the knot grows it eventually cuts off the flow of water and nutrients to the branch, causing stunting, wilting, and dieback. Gradually the entire tree may weaken and die if the severity of the disease increases.
Unfortunately it is very difficult to control. Pruning is the most important control measure. Research has shown pruning can reduce infection by 80%. Prune off all knots in late winter or early spring before growth starts. Bury, burn, or compost all discarded plant material. Pruning cuts should be made at least four to eight inches below any swellings or knots because the fungus grows beyond the edge of the knot. Pruning tools should be sterilized between cuts using 1 part bleach to 4 parts water.
Q: I have a group of Viburnum shrubs in the front of my home that are getting too large, and no matter how low I cut the branches, new shoots arrive in the spring and grow as tall as they once were. If I cut the new shoots to half height then they double. How should I manage this type of shrub?
A: Pruning is a process that not only removes wood, it encourages the growth of new wood. Try taking some of the stems down all the way to the ground. Then new growth will come from the base of the plant and will take a little longer to get tall. This will also open the plant to more light which will courage development of more flowers on the remaining stems. If you still cut stems back half way, be sure to cut directly above a bud. Stubs occur when our pruning cuts are not close enough to the next bud.
Q: How often should older trees be pruned and when is the best time?
A: Winter can be a great time to prune while trees are dormant, but ultimately pruning depends on the age of the tree and goals for pruning. When trees are young, we are trying to develop a good form and strong structure and will likely be pruning yearly. After a tree has been in the landscape for a few years, hopefully the structure and form are set, and we turn to removal of problem branches and repair of damage so pruning may not be needed every year. With older, mature trees it would be best to prune as needed. Basically that would involve looking at all the trees each year and only pruning the ones that have a need (storm damage, broken limbs, rot, disease, and crossing limbs). Talk to your arborist and see if he will give you an estimate on that type of plan. The Illinois Arborist Association has a website that lists companies with certified arborists (www.illinoisarborist.org).
Q: I’m noticing needle and leaf browning much more than usual on my evergreen holly, yews and boxwoods this winter. Should I prune it off now before the new growth begins in the spring?
A: When there is extremely cold temperature in winter we expect to see some tip die back (winter burn) and browning on evergreens, including broadleaf evergreens like holly and rhododendrons. Winter burn occurs when evergreens naturally transpire but cannot replenish lost moisture because the ground is frozen. Smaller plants fair better if there has been adequate snow cover to insulate them.
Be patient and wait to see how well the new growth emerges before doing any severe pruning. The new buds for this years’ growth on evergreens are in the tips of the branches, on holly they are hidden where there is a leaf attached. Plants surprisingly can withstand extreme cold and will push out new growth once winter has lessened it grip. Most evergreens start to show growth sometime in early to mid-May.
Q: Is there a point when an emerald ash borer-infested ash tree should be cut down. I know treatment is futile after 40% crown die back, but I was wondering how long an infested tree can remain standing before it poses a danger.
A: Great question. Once a tree is past the stage of successful treatment, the question "Why keep it?" must be asked. If the tree is past treatment, it will be in decline and most likely die in a year or two. We have learned that ash trees contain very less water content, than say an oak, so when an ash tree is dead it becomes brittle with the possibility of becoming a hazard tree. The infested tree also contains a population of developing larvae. Removing that tree during the dormant season gives the village the opportunity to reduce the population and destroy the tree at a time when those insects cannot spread to other trees. Reducing the population may not have a big impact in heavily infested areas, but could be useful in areas where the insect is less prevalent.
Q: We have a 5 year old red twig dogwood hedge (Cornus sericea ‘Cardinal’) that is dying one or two stems each year. The older stems are turning dark brown, almost black then dying off completely. Is there a way in which to control, if not eradicate this issue? Would it be safe to replant with the same cultivar or are they too susceptible?
A: What you are describing sounds like a stem canker. There are no sprays to control cankers. Remove the infected stem to the ground in dry weather. Be sure to sterilize tools between cuts so the disease is not spread to a healthy stem. Red twig dogwoods benefit from regular pruning. Pruning older stems promotes new growth, which has the brighter red color.
Q: The bark on our new maple has a long crack developing down the trunk. Should we wrap the trunk to protect it from further injury?
A: This winter injury is known as frost cracking. The bark of young, thin-bark trees can split open as a result when temperatures drop quickly. It occurs most commonly on the south or west side of the tree where the sun warms the bark causing expansion, then temperatures drop and the bark contracts more rapidly than the live wood below. A vertical split is the result. It becomes more obvious in the spring when the tree trunk starts to put on girth. There is no treatment for bark splitting. The wound typically heals itself but can be vulnerable to disease as it heals. Keep the tree healthy by watering in dry periods. Wrapping is not recommended.
Q: Why are my white pine's needles turning brown and dropping off?
A: This seasonal needle loss, also called fall needle drop, is a natural occurrence triggered by weather and seasonal changes. Evergreens do not hold their needles forever; as they age, they will turn yellow, then brown, and then drop off. The change can be gradual or, with some species, quite rapid. It does not always occur every year. As long as needle drop is restricted to older growth and is not excessive, it is a normal and natural process.
Q: Can I prune my oaks and maples this fall?
A: You may want to wait until a bit later in the season to prune your trees. In early fall, the wounds caused by pruning close more slowly than at other times of year. For most trees, the best times for major pruning are in late fall (when plants are dormant) and again in winter to early spring because wounds heal faster. Some plants, such as maple, elm, birch, and walnut are considered “bleeders” and should not be pruned in early spring when sap begins to rise, hold off on these trees until the leaves have emerged.
Q: How do I prune my hydrangea?
A: It depends upon which type of hydrangea you have. Hydrangeas do not need to be pruned every year. Hydrangeas that flower on the old wood, such as oak-leaved hydrangea (Hydrangea quercifolia) and bigleaf hydrangea (H. macrophylla, such as Endless Summer® and Twist n Shout®), should be pruned right after flowering or during the summer. Annabelle (Hydrangea arborescens) and panicled hydrangeas (H. paniculata, such as Limelight) flower on new wood produced the same year, so they can be pruned in the late winter or early spring before they bloom. Avoid summer pruning or you will remove the new flower buds. Use the heading back technique to prune at the appropriate time of the year.
Q: Can I prune back my clematis now that the season is over?
A: Clematis is divided into 3 basic categories to determine pruning times:
- Group 1 (Type A) is typically a species clematis (C. alpina and C. macropetala and their cultivars) that flower in early spring. These vines produce their flower buds on the previous year’s stems. They can be pruned to keep them within their growing space, or to remove dead and unsightly foliage. They usually do not require pruning, but if needed, prune right after flowering is finished.
- Group 2 (Type B) is the large flowered hybrids. This type normally blooms in the spring and possibly again in the fall. All of the clematis in Group 2 bloom on 'old wood' (actually on short shoots from old wood) and should not be pruned except for removing deadwood in early spring after the leaf buds have begun to open slightly.
- Group 3 (Type C) are the popular summer-blooming varieties, such as the C. viticellas, C. jackmanii types; the herbaceous species, such as C. integrifolia and C. recta, and the late bloomers such as sweet autumn clematis (C. terniflora). They all produce flowers on new wood in the current year and should be pruned back in late winter to early spring to about 12 to 14 inches above the ground. Leave at least two pairs of buds (4) on each stem of the plant to insure new growth.
Q: Many of my trees and shrubs suffered from last year’s brutal winter. Is there a way to protect them this year?
A: All plants, especially conifers and broadleaf evergreens (e.g., boxwood and rhododendrons), are susceptible to winter desiccation from drying winds. Be sure plants have adequate soil moisture before the ground freezes. Shading plants from winter sun and airborne salt spray can also be helpful. Fasten burlap or plastic to stakes on the exposed side of plants. To prevent broken branches from heavy snow load, gently brush fresh snow from the branches before it freezes. Allow ice-encrusted branches to melt off naturally.
Q: Can I divide / move ornamental grass in the fall?
A: Cool-season grasses such as feather reed grass (Calamagrostis) can be divided from late winter to early spring or from late summer to early fall. In the upper Midwest, spring division is the most reliable. Warm-season grasses such as fountain grass (Pennisetum) and silver grass (Miscanthus) should be divided when they are in active growth; late spring / early summer is the ideal time. Fall division of warm-season grasses is generally not recommended.
Q: What are the black circular spots covering my maple leaves?
A: Those spots are a symptom of tar spot, a fungal leaf disease. It is primarily a cosmetic disease that will not kill the tree. Initial infection occurs in the spring and appears as small, yellowish spots. As the season progresses, the spots enlarge to roughly ¾ inch in diameter and the center becomes raised and turns black, resembling a glob of tar on the leaf surface. To reduce or eliminate tar spot, simply remove fallen leaves from the ground and discard them.
Q: Is there a way to protect my plants from animal damage this winter?
A: One of the best preventative measures to protect plants against animals is to construct a cylinder of hardware cloth, chicken wire, or fencing around plants to protect against the rabbits, mice, and deer that damage the bark and chew on twigs. Set up the fencing in the fall so stakes or support can go into the ground easily before the soil freezes. More persistent problems may require more vigorous management.
Q: How late is too late to plant bulbs?
A: Spring flowering bulbs are planted in the fall, ideally in September and October, to allow them to establish roots before the top growth begins in the spring. Planting too early may cause bulbs to sprout leaves, which are then damaged by winter. Planting too late may not give the bulb enough time to develop adequate roots. If it is November and the ground is not yet frozen, go ahead and plant your bulbs. Water them well to settle the soil and then cover the soil with a thin layer of mulch or ground-up leaves to insulate the soil. If you cannot dig holes, consider planting them in containers to force indoors later. The planted containers will require a 10- to 13-week cold period (an unheated garage works well) to trigger bulbs to grow roots and initiate flowers.
Q: Is it difficult to grow an oak from an acorn?
A: It depends on what type of oak you are trying to grow. Acorns from oaks in the white oak group (white, bur, swamp white) ripen in one year and will germinate as soon as it is planted in the fall. The oaks in the red oak group (red, pin, Hill’s) require two years to reach maturity before they will germinate.
Q: Can I use oak leaves as mulch around my trees and shrubs or are they too acidic?
A: The idea that oak leaves are too acidic to use as mulch is an old myth that you can happily ignore. Although oak leaves do contain tannins, the tannins are only slightly acidic and will not affect the growth of mulched plants. In fact, leaves of all kinds can be a gardener’s best friend. In nature, fallen leaves remain on the ground where they break down with the help of soil organisms, returning organic matter back to the soil. These nutrients act as a natural fertilizer for trees and shrubs. Decomposing leaves used as mulch also help to insulate the soil from extreme temperature fluctuations, increase the soil’s capacity to hold moisture, and eventually provide a more favorable root environment.
It helps to mow or shred leaves before using them around your trees / shrubs or adding them to the compost bin. This makes the leaves decompose faster and prevents the leaves from forming a tight mat that prevents water from interacting with the plants’ root systems. For best results, do not apply any more than 3 to 4 inches of leaf mulch.
Q: I notice fuzzy little round growths on my oak leaves every year. The tree looks healthy otherwise. Should I treat this problem?
A: No treatment is necessary. What you see are plant galls, which are swollen, abnormal growths that appear on the leaves, twigs, and flowers of many plants. They will not seriously affect the health of your oak. Most galls are caused by chemicals produced by small insects, such as mites or aphids, during egg-laying or feeding activities on the leaf buds. These chemicals cause plant cells to swell, creating the galls. The insect may live inside the gall for a brief period before completing its life cycle.
Q: How do I get rid of Japanese beetles?
A: A mild infestation of Japanese beetles can be controlled by picking them off the roses and dropping into a container of soapy water. A strong spray of water into trees can get them to move elsewhere. Avoid using pheromone traps; Japanese beetles are flying insects and the traps will bring in more beetles than they catch. Chemical sprays for Japanese beetle can be harmful to beneficial insects and not recommended. Soil insecticides provide short-term control if applied mid-August through September.
Q: My landscaper recently mulched my garden beds but it’s about 6 inches deep in many areas. Isn’t this too much?
A: Ideally mulch should only be a 1-3 inches deep on the soil surface above the root system. Applying more than 4 inches may injure plants by keeping the soil too wet and limiting oxygen to the plant roots. Keep mulch away from base of plants, and do not let it rest against the bark. Around individual plants mulch should form a donut shape--not a volcano.
Q: The leaves of my red maple appear to be smaller and pale green in color. Is there something I can do to correct this problem?
A: The yellowing of leaf tissue due to the lack of chlorophyll, the green pigment in leaves, is known as chlorosis. Symptoms of chlorosis can vary depending upon the cause, but generally leaf tissue will turn a yellow-green while the veins remain green. Symptoms can appear over the entire tree, but may be more severe on one portion of the tree than another.
A major cause of chlorosis in the maples is a deficiency of manganese. Although not actually lacking in the soil, these nutrients become unavailable to plants where the soil is heavy and alkaline (pH above 7.0). Other possible causes of chlorosis include poor drainage, damaged roots, grade changes, or compacted soil.
Management for chlorosis symptoms varies with the cause, but the most lasting results are obtained by improving the tree’s rooting environment. Apply a one-to-two inch layer of organic compost (e.g., leaf mold) to the soil surface, followed by three to four inches of organic mulch (e.g., wood chips). Be sure to water during drought periods to minimize stress.
While there are several effective treatments available, they may have to be used annually. Soil fertilization treatments produce the best results, but are usually the slowest to respond. Soil treatment is best done in early spring through mid-May. For mildly chlorotic trees, fertilize with a nitrogen or nitrogen- and sulfur-based fertilizer.
DED-resistant elm cultivars
Q: Our neighbor recently had their American elm removed after being diagnosed with Dutch elm disease. We recently planted one of the resistant elms, should we be concerned about our tree?
A: Dutch elm disease (DED) is one of the most serious diseases to affect American elm (Ulmus Americana) and has killed thousands of elms across the United States. Prevention and early detection continue to be the keys to protecting elm trees from DED. Many of the new cultivars available in the nursery market are resistant but not immune.
The disease is caused by the fungus Ophiostoma ulmi. It can be transmitted by two species of the elm bark beetle carrying the fungus from an infected to healthy tree, or transmitted through root grafts between trees growing within close proximity to one another. Because elms are most susceptible during the spring and early summer, when bark beetles are present, avoid pruning during this period to reduce the risk of contacting DED. Maintain good tree maintenance practices such as thoroughly watering during extended periods of drought, providing a three-to-four-inch layer of compost and mulch around the base of trees, fertilizing, and periodic pruning to ensure your tree remains healthy and vigorous.
Q: What is the proper way to water my trees and shrubs?
A: Generally speaking, plants get most of their water and nutrients from the top 12 inches of soil. The key is to keep roots moist, but don’t overdo it. Let the top few inches of soil dry out between watering so that the roots and soil organisms can breathe.
Newly planted trees and shrubs need through watering after initial planting and must be watered regularly until the roots have properly developed (2 to 3 years). Concentrate on watering the root ball. Place a garden hose at the base of the plant and let it run slowly, soaking the soil to a depth of 2-3 inches. Watering too much or too little can result in injury.
Established trees and shrubs will require supplemental watering if we have not received adequate rainfall. Trees and shrubs should receive at least 1-2 inches of water every 7 to 10 days throughout the growing season, more during an extremely dry period. Water plants slowly, deeply, and thoroughly under the canopy zone (from the trunk to the ends of the branches) rather than daily.
Viburnum crown borer
Q: The stems of my arrowwood viburnum are dying one branch at a time. I’ve also noticed cracks near the base of the plant. What’s going on?
A: These sound like the symptoms of viburnum crown borer. Viburnum crown borer adults are clearwing moths that emerge from May to August, and closely resemble wasps. They typically lay eggs in cracks on the stem about 12 inches above the ground. Small holes and the presence of a sawdust-like material are signs of a borer. When the eggs hatch, young larvae tunnel downward into the base of the plant, eventually killing the stem. Stressed plants are more susceptible to borer attack, so try to keep plants healthy by applying mulch around the base and watering well during dry periods. Chemical sprays are available and should be applied in June when wasps are active.
Q: There are a number of pinkish-white bumps all over the stems of my magnolia tree. They seem to be getting larger. What are they and how do I treat them?
A: Your magnolia tree has magnolia scale. Scale insects are sucking insects that produce a sap which often causes leaves to turn black from sooty mold. Heavy infestations can cause twig dieback and kill branches. The adult scale continues to grow throughout the summer. They are covered with a white, mealy wax. The adult mother scale will give birth to young larvae, called “crawlers” in late August into mid-September. The best time to kill the magnolia scale is when the young crawlers are present. There are a number of products available, including seasonal horticulture oils, insecticidal soaps, and chemical insecticides.
Q: My arrowwood viburnum leaves are full of small holes. What is chewing on them?
A: This may be the damaged caused by viburnum leaf beetles. The larval stage of the beetle feed on new leaves for two-to-three weeks, causing numerous holes in the leaves. When mature, the larvae crawl to the ground, usually in mid-June, and pupate in the soil. Adults emerge from the soil (early July) and also chew on the leaves. Their feeding damage forms irregular round holes in the leaves. The beetles are about ¼ inch long and generally brown in color.
The adult beetles will be mating and laying eggs from summer into fall. There is one generation of the beetle each year. Heavy and repeated defoliation by the viburnum leaf beetle can lead to death of the shrubs. Insecticides can be used on the larvae in May/June when they are feeding and on the adults in summer when they are feeding.