Selection, planting and care of trees and shrubs to avoid the need for pesticides

David Smitley<smitley@msu.edu>, MSU Entomology; Diane Brown, Rebecca Finneran<Finneran@msu.edu> and Erwin Elsner, MSU Extension; Joy Landis, MSU IPM; Paula Shrewsbury, Univ. of MD Entomology; Daniel Herms, The Davey Tree Expert Company, Kent, OH; and Cristi L. Palmer, IR-4 Project-Rutgers - May 01, 2019

The best way to minimize pollinator exposure to pesticides is to create and maintain healthy landscapes with plants that rarely require a pesticide application.

A shady lawn. — Check for variability in soil moisture and light and select plants accordingly. Photo by Rebecca Finneran, MSU Extension.

The best way to minimize pollinator exposure to pesticides is to create and maintain healthy landscapes with plants that rarely require a pesticide application. Choose perennials, shrubs and trees that are adapted to your climate and soil type. Make sure they are winter hardy in your area and will get the amount of sunlight they require. The most important considerations needed to establish healthy plants are covered in the seven categories that follow.

1. Do not choose plants known to have major pest or disease problems.

The best way to avoid the need for pesticides is to choose pest-resistant plants. Review the problem-prone plants in the table on this page before buying plants. In many cases, resistant cultivars or alternative plant types with the same characteristics are given. There are many guides available for choosing other trees and shrubs as alternatives to the problem-prone trees listed below (See Cregg and Schutzki 2006).

Problem-prone plants likely to need insecticide or fungicide treatment to remain healthy
Tree common name	Scientific name	Potential problems of concern
Ash	Fraxinus spp.	All North American Fraxinus spp. (ash) are susceptible to emerald ash borer, and once the borer is present they will not survive without insecticide treatments.
Austrian pine	Pinus nigra	Susceptible to Diplodia tip blight and Zimmerman pine moth.
Boxelder	Acer negundo	A weak-structure tree that hosts boxelder bugs, a nuisance in houses. Seeds and seedlings may be a problem.
Colorado spruce	Picea pungens	Prone to fungal cankers that kill the lower branches, particularly when drought-stressed.
Common lilac	Syringa vulgaris	Prone to powdery mildew, scale insects and bacterial blight. Look for blight-resistant cultivars.
Crabapple	Malus spp.	Many cultivars susceptible to leaf drop due to apple scab. Plant scab-resistant cultivars.
European mountain ash	Sorbus aucuparia	Susceptible to scab, fungal cankers, scale insects and borers.
European white birch	Betula pendula	All European and Asian cultivars are highly susceptible to bronze birch borer. Use native birch species when possible.
Poplars	Populus spp.	Fast growing, but susceptible to cankers, galls and borers.
Purpleleaf plum	Prunus cerasifera Prunus x cistena	Susceptible to borers, cankers and blacknot.
Russian olive	Eleagnus angustifolia	Branch dieback from cankers and verticillium wilt. Somewhat invasive.
Siberian elm	Ulmus pumila	Fast growing, but short-lived and brittle. Not tolerant of shade. Variable level of resistance to Dutch elm disease.
Willow	Salix spp.	Brittle and susceptible to crown gall, cankers and borers.
Wintercreeper euonymus	Euonymus fortunei	Highly susceptible to euonymus scale. Unlikely to survive without treatment for scales.

2. Make sure the plant you are considering is winter-hardy in the climate zone where you live

This is easy to do using the USDA Plant Hardiness Zone Map.

This website has an interactive map of plant hardiness zones for all of the U.S. Perennials, trees and shrubs at the garden center should have the cold-hardiness zone listed on the tag of each plant. If your area is listed as zone 5b, for example, make sure the plants you buy have zone 5b or a smaller number listed (smaller numbers are more cold-hardy).

3. Check your soil pH

Appropriate soil pH levels are often listed on plant labels. Make sure your soil pH falls within the range considered adequate for the tree type you are considering. Some classic examples of trees sensitive to high pH soils are red maple, pin oak and white pine. The foliage may begin to turn yellow (chlorosis) when trees grow in soil with a pH above 7.5 due to a lack of iron or manganese. These deficiencies are caused by tree roots failing to absorb these metals from high pH soils. To learn your soil’s pH, you will need to send a sample to a soil lab. The MSU soil lab (http://www.spnl.msu.edu/) offers a kit for collecting and mailing soil samples and will return customized results on a variety of soil characteristics. Order E3154 at shop.msu.edu.

Check for variability in soil moisture and light and select plants accordingly. Photo: Rebecca Finneran, MSU Extension. — Check for variability in soil moisture and light and select plants accordingly. Photo by Rebecca Finneran, MSU Extension.

4. Check light and moisture requirements

Some trees and shrubs do not grow well in shady sites, and some actually prefer some shade. Find light and moisture requirements on the plant’s label and make sure it will receive enough hours per day of sunlight to thrive.

5. Allow enough space for the root system and branches to grow to full size

Note the maximum height listed on the plant label and make sure this is appropriate for the site. Trees and shrubs need room for roots to grow down and outward from the trunk. In general, small trees should not be planted any closer than 8 feet from a building, and large trees no closer than 15 feet away, although 20 feet or more is better.

6. Prepare planting sites properly

Dig a hole at least twice as large as the root ball and fill with the soil that was removed amended with no more than 5% organic matter. Heavy clay soils can be amended by adding some sand or silt. Make sure water drains well from the planting site. Unravel roots before planting and place the root ball so the area where the roots start to flare outward from the trunk are level or 1-2 inches above the surrounding soil surface.

7. Avoid drought stress

The most common plant stress is caused by drought. Because we see flowers wilt when they lack water, people understand the importance of frequently watering flower beds. Trees, shrubs and perennials are also weakened by prolonged water stress. In fact, adequate soil moisture may be the most important factor in maintaining ornamental plants. Drought stress encourages insect and disease problems that may then require a pesticide. Water new trees as directed on the plant label.

Read the next article in this publication series: How to control invasive pests while protecting pollinators and other beneficial insects

Or return to the beginning of this publication: How to protect and increase pollinators in your landscape

This article was published by Michigan State University Extension. For more information, visit https://extension.msu.edu. To have a digest of information delivered straight to your email inbox, visit https://extension.msu.edu/newsletters. To contact an expert in your area, visit https://extension.msu.edu/experts, or call 888-MSUE4MI (888-678-3464).

MSU Extension Pollinators & Pollination