By Dave Sutton, ISA Board Certified Master Arborist
Trees and shrubs are important components of most landscapes, adding to the beauty and value of property. Here we discuss some basic principles of care to maintain the health and beauty of landscape plantings and native trees. Homeowners with the interest and energy can do much tree and shrub care for themselves.
Most native trees and well established ornamentals will never need spraying. It is estimated that 90 per cent of the spraying done is unnecessary, unwarranted and may be harmful. The all-too-common practice of “cover sprays”, or spraying all the trees and shrubs at the same time with the same chemicals is never appropriate. This “one size fits all” approach results in excessive chemicals added to the environment, damage to non-target objects and organisms, and seldom provides any beneficial effect on the target plants.
The business model of many “spray-sellers” requires many trucks applying pesticides to all plant material on many properties. This is not a scientifically sound approach and results in excessive pesticides applied to the environment.
Sprays should be specific both in type and timing. A problem or potential problem should be identified and then treated with a specific application with precise timing based on the life-cycle of the pest or disease being treated. Soil applied or trunk injected systemic pesticides may be a better treatment approach for many tree problems. Some treatments can be applied by homeowners but should be guided by a certified professional.
A certified professional can provide a Landscape Health Prescription that identifies trees and shrubs in the landscape, details evidence of pest and disease damage and potential future threats. Advice for preventive treatments or curative treatments can be documented. Some treatments can be done by the homeowner, some may need professional care.
Trees and shrubs thrive best if planted in appropriate locations and if they receive proper care and maintenance. Considerations must include amount of sun, type of soil, presence of alternate host species, drainage, pest and disease threats, etc. For example, red maples and pin oaks do not thrive in heavy clay soils that have a high pH level. Many trees and shrubs do not thrive in partial or heavy shade. Pines and other evergreens often do poorly on berms. Pines and spruces that are crowded are more likely to get needlecast disease. Organic mulch around trees can protect them from temperature extremes, prevent mower and weedwhacker damage to the trunk, and add organic content to the soil as the mulch decomposes.
Many people do not think about trees needing to be watered, but during long dry spells or seasons with less than normal precipitation, watering trees may be beneficial. Especially ornamentals planted within the past two to three years will benefit, because they may not have developed enough good roots to sustain them through periods of diminished rain
SPECIFIC PESTS AND PROBLEMS.
EMERALD ASH BORERS are a threat to all ash trees and excellent methods are available to preserve ash trees. Specifically TREE-Age chemical is proven to give excellent control when applied by trunk injection. TREE-Age (emamectin benzoate) is a restricted use pesticide and must be applied by a professional. Small ash trees (under 8 inch trunk diameter) can be treated with soil insecticide (see bronze birch borers below) but treatments must be done yearly, whereas emamectin by trunk injection gives two years of control with one injection. Go to this link to learn more about treatment options:
BRONZE BIRCH BORERS can damage and/or kill birch trees not protected with systemic insecticides such as imidacloprid. Sprays are poor alternatives to systemic controls for either type of borer. Homeowners can apply soil drench of imidacloprid for small ashes and birches. Imidacloprid is available in garden centers as a ready to use product. This product is mixed with a bucket of water and poured around the base of the tree. Be sure to follow label instructions for safe and effective use. iF A BIRCH TREE IS ALREADY INFESTED WITH BORERS, IT MAY BENEFIT FROM INITIAL TRUNK INJECTION OF EMAMECTIN TO KILL EXISTING BORERS AND THEN CAN BE MAINTAINED WITH ANNUAL SOIL DRENCH OF IMIDACLOPRID.
- APHIDS rarely cause significant problems for trees and rarely need any treatment. They do drip some honeydew when they feed and that can be a nuisance to the humans who live below the trees. Many of the trees that are sprayed commercially, supposedly to control aphids, will never have aphids even if not sprayed. On bushes, the aphids seen are usually on the newest growth and can be removed by simply trimming the bush. Aphids can also be blasted off of plants with a stream of water if they aren't protected by curled leaves. Soil applied imidacloprid can give control if applied before the aphid buildup.
- SCALE INSECTS can cause large amounts of sap drippage from trees and black mold grows in the sweet sap residue. Scales can cause lower branch dieback, but are usually more of a nuisance problem. Systemic treatments (soil or trunk) are usually best for this problem. Sprays can actually make the problem worse by killing predator insects. In particular, sprays during May, June and early July for cottony maple scale or lecanium scale will not kill the scales but will kill the twice-stabbed ladybugs and parasitic wasps that help keep scales under control. HOMEOWNERS can apply soil drench of DINOTEFURAN for quick results supplemented with soil drench of IMIDACLOPRID for longer lasting scale control on small trees. Large trees with serious scale problems will need professional application of one or more insecticides by trunk injection and/or soil injection.
- MITES can cause problems in certain plants and should be treated with selective miticides, never mixed with insecticides, which kill the natural predator insects that help keep mite populations down. Most mite build-ups are caused by excessive insecticide use, so avoiding insecticide sprays is important. There are some plants that commonly get warm season mite build-ups and a selective miticide spray in mid-summer may give good control of these mites and avoid the early leaf drop or leaf distortion caused by mites. Unfortunately, selective miticides are not commonly available to homeowners. Malathion can be used, but does kill some of the predator insects. Summer oil spray could be used with care, applied on cool mornings. Burning bushes, boxwoods, and Alberta spruces are common hosts of warm season mites. Mites can also be blasted off of plants with a strong stream of water, giving the plant a thorough early morning bath once or twice a week, avoiding the use of any pesticides.
- LEAF-EATERS AND LEAF MINERS. There are a number of caterpillar pests that eat leaves or needles, including gypsy moths and canker worms, as well as tent caterpillars in the spring and webworms in the late summer. There are also leaf-eating beetles, especially Japanese beetles, that can cause significant defoliation. When infestation is heavy, sprays or systemic treatments may be warranted. Small ornamentals can be sprayed by homeowners with ready to use insecticides. Large trees will usually require professional equipment. Japanese beetles are best controlled with soil applied systemics. Imidacloprid gives some control. Acephate can be soil injected by professionals and gives excellent control, even on large trees.
- GALL INSECTS cause abnormal growth of plant tissues, usually more of a cosmetic problem. Common are twig galls of spruce, leaf galls of maples, woody galls of oaks. Sprays or systemic treatments may be helpful if the galls are killing twigs or small branches or if the cosmetic damage is unacceptable. Soil applied imidacloprid is effective for many of these galls.
- BORERS. See above in first paragraph of this section. The Emerald Ash Borer is the most prevalent life-threatening tree problem in Michigan and much of the midwest, so it was addressed first in this section.
- WHITE PINE WEEVIL. This insect kills the top leader in white pines and spruces. Control requires a spray of insecticide to the very top part of the tree in early April or application of soil systemic like imidacloprid in the fall. If spraying, only the top quarter of the tree needs to be sprayed, but it should be sprayed thoroughly before April 15th in northern Michigan and even earlier in southern Michigan. If trees are small enough, a homeowner can do this with a hose end sprayer.
- FOLIAR DISEASES. The most common diseases in landscapes are fungal diseases of pines and spruces (needlecast and tip blight). Also commonly seen are leaf spot and scab of flowering trees. These diseases are best treated with specific fungicide sprays, done with specific technique and timing. A series of sprays starting in mid-April is necessary to control tip blight and leaf scab. Two sprays, one in mid-May and one in mid-June are needed for needlecast disease of pine and spruce. Homeowners can apply fungicide sprays to small ornamentals, but different problems require different fungicides and specific timing. Specific pesticide recommendations are found at the end of this article.
- WILTS. Dutch Elm Disease is a mortal threat to elm trees, but can be prevented with injected fungicides. Oak wilt may be prevented and/or treated with injected fungicides. Oaks and elms should not be trimmed during warm months because the open wounds may attract the insects that carry these diseases. If untimely trimming of these trees occurs (storm damage clean-up, etc.) all wounds should be sealed.
- CHLOROSIS. Yellow, pale leaves on red maples, pin oaks and river birches are usually a symptom of nutrient deficiency because of heavy alkaline soils. The best approach is to avoid planting these trees in heavy clay soils. If you have them already, you can apply elemental sulfur to the soil to help acidify it. Also, chelated micro-nutrients can be applied. Large trees can be treated by professionals with trunk injected nutrients.
- ARMORED SCALES. These include euonymus scale and pine needle scale and several others. Soil drench with a product containing dinotefuran can give excellent control of these scales.
SPECIFIC CHEMICALS FOR SPECIFIC PROBLEMS
Needlecast (Rhizosphaera) on Colorado Blue spruce: Sprays should be applied when new growth is half extended and when fully extended (usually mid-May and mid-June in Michigan). Fungicide should have active ingredient chlorothalonil. Copper fungicide can also be used. Mix as directed and apply with hose-end sprayer to new growth, drenching it to the point of runoff. It is best to add an adjuvant, specifically a spreader/sticker, to help the product spread evenly on the needles and to keep the product in place. Spray the lower part of the plant thoroughly up to a couple of feet above the affected branches.
Needlecast on Austrian Pine: Sprays should be applied as above, but fungicide should be a copper fungicide labelled for Dothistroma needlecast (we use Junction). There is a copper fungicide (Bonide brand) available at some garden centers. Mix as directed and apply with hose-end sprayer to new growth, drenching it to the point of runoff. It is best to add an adjuvant, specifically a spreader/sticker, to help the product spread evenly on the needles and to keep the product in place.
Diplodia tip blight on Austrian Pine: Sprays should be applied just before bud break and every 10 days to two weeks for two additional sprays, using any fungicide labelled for this problem. One such fungicide is propiconazole. Other directions above also apply. Mix as directed and apply with hose-end sprayer to new growth, drenching it to the point of runoff. It is best to add an adjuvant, specifically a spreader/sticker, to help the product spread evenly on the needles and to keep the product in place.
Here is a link to an Iowa State information page that discusses treatment of these fungal diseases of conifers:
Scab on crabapple; Sprays should be applied just before or during bud break, then again just before the blossom, then just after blossoms fall using a fungicide labelled for this problem. We have found best results using a systemic fungicide for the early sprays. Look for one with active ingredient propiconazole. If you cannot find that, there are other fungicides labelled for scab, such as those with active ingredient chlorothalonil. Mix as directed and apply with hose-end sprayer to new growth, drenching it to the point of runoff. It is best to add an adjuvant, specifically a spreader/sticker, to help the product spread evenly on the plant tissues and to keep the product in place.
Insecticides: Many problems can be controlled with soil drench of imidacloprid, available under several brand names. Just look for active ingredient imidacloprid, usually at a 1.47% concentration. Another good systemic insecticide is active ingredient dinotefuran. There is a homeowner product available with dinotefuran- Ortho Tree and Shrub Insect Control (but be sure to check that the active ingredient is dinotefuran.)
There are also several over the counter sprayable insecticides, including Sevin, Malathion, and synthetic pyrethroids like bifenthrin, deltamethrin, pyrethrum, etc. As discussed above, sprayed insecticides can and should be avoided in most cases.