University of Vermont Extension
Department of Plant and Soil Science

Summer News Article


Contact: Dr. Leonard Perry, Extension Professor
University of Vermont


Reaching for the bottle is not the solution to controlling insect pests in perennial gardens.

Why? First, chemical pesticides don't always hit the intended target. Many gardeners use the wrong product or apply it at the wrong stage of development of the pest. Second, pesticides are toxics. Application can upset the delicate ecological balance, destroying more than just the targeted pest.

Insect pests aren't always to blame for poor growing, unhealthy perennials. Sometimes the problem lies in poor site selection or inadequate preparation of the beds.

One of the top mistakes gardeners make is to plant in compacted soil, which is often the problem when landscaping a new home. When bulldozers and other heavy equipment move over the soil, it becomes compacted. That's why it's important, if possible, to speak with your contractor early and try to work out a plan.

Poor soil drainage is another serious problem. Roots can't breathe, so plants die. Raised beds, especially in heavy clay soils, is one solution. Build the soil by adding peat moss.

However, even loam or sandy soils benefit from the addition of leaf mulch, peat moss, compost, or other organic matter. Application of a slow-release fertilizer in spring or early summer will improve the long-term health of the plants. Use wood ashes sparingly; otherwise, the pH of the soil will be too high (excess potash). You also need to be careful of what was burned.

Avoid planting perennials near trees as they'll compete with the roots for nutrients. Don't plant under pine trees as pine needles tend to lower the pH below what most plants like.

When selecting plants, search out hardier varieties to limit the need for chemicals for pest control. Professional nurseries should know what's hardy for your area.

You'll also want to buy varieties with proven low resistance to disease. For example, "David," a vigorous white phlox, and "Mahogany," a deep red bee balm, are good mildew-resistant plants.

When buying lilacs, again ask regarding resistant varieties. Canadian hybrids with their bigger, looser flower clusters tend to be "tough." Although not as attractive as the European varieties, they won't succumb to mildew problems.

If your perennials become diseased, stop the spread by cutting them down and getting rid of them, preferably by burning. Don't add them to your compost pile as you'll be reintroducing the disease when you spread the new compost next year.

If you need to use pesticides, first reach for organic products such as rotenone or pyrethrum, which will break down after use. But think it over before you use any insecticide, keeping in mind that other organisms may die, not just the intended pest. And just because a product is "organic" doesn't mean it's safe. Use the same precautions as you would with synthetic pesticides.

Horticultural oils are effective for controlling many pests, especially on delphinium, asters, and phlox, providing you spray only when the insect is present and don't broad base your applications.

For soft-bodied insects like slugs, you can use horticultural diatomaceous earth (available at garden centers) for control. Some gardeners have luck with homemade remedies such as salting the slugs or setting out pans of stale beer. Copper strips work best.

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