Many of the spring and summer perennials can begin to look a bit scraggly at this time of the year due to insect munching, disease, senescence, or simply from the natural weathering of the plants over the season. Many gardeners wonder what they can do to improve the look of their gardens. Is okay to cut some of these "battle scared" plants back?
The other day we had a call from a gardener who had powdery mildew on her peonies and she was wondering what she could spray to control this. Sometimes towards the end of the season it's not worth the time or expense to spray for disease control on herbaceous perennials. It's often just better to cut them back to the ground. In general, we recommend that you wait until the peony foliage begins to yellow in early fall before it is cut back. However, when the foliage is covered with powdery mildew, it cannot photosynthesize efficiently so in this case, it is fine to cut it back even as early as mid August.
Diseases like powdery mildew often become more prevalent in the late summer when plants tend to be under more stress or are beginning to decline naturally. Plants that are in a weakened state are more likely to be attacked by fungal diseases and insects. We often receive calls in August and September about mildew on summer phlox and peonies, rust on hollyhocks and asters, and leaf spots on many other perennials. These perennials can be cut back severely if they become an eyesore in the garden. Even some trees and shrubs like crape myrtles can be pruned to remove just the diseased portions of the plant. This diseased foliage should always be bagged up and discarded in the trash; never put it in your compost pile because they often don't get hot enough to destroy the disease organisms. Many perennials, including phlox and daylilies, will regrow fresh new foliage for the fall season.
This year powdery mildew and downy mildew seems to be exceptionally bad in the vegetable garden, especially in the cucurbits; squash, melons, and cucumbers. Affected plants decline rapidly and eventually die back completely. These vines should be removed from the garden, bagged up and disposed of. Remember to rotate your crops next year!
If you have had disease problems in the flower garden or vegetable garden, one of the best ways to avoid these problems the following season is to carefully rake up and remove all dropped leaves, fruits, and cuttings from the garden in the fall. Fungal spores overwinter in this plant debris and will reinfect the plants when they emerge in the spring.This is especially important in the vegetable garden. Another way to reduce disease in the garden is to plant disease resistant varieties whenever possible. There are many new cultivars and hybrids of trees, shrubs, and perennials that show resistance to many of the common fungal diseases. Also be sure to rotate your crops in the vegetable garden if you are able.
Weeds can act as both reservoirs and alternate hosts for a wide variety of insect pests and diseases that plague our gardens. This is one of the major reasons that it is important to keep your gardens and also the adjacent areas as weed-free as possible. Weeds harbor insects like leafminers, whitefly, slugs, aphids, thrips, flea beetles, and stink bugs. Some of these insects transmit viruses to our plants in addition to being destructive to the plants directly.
There are many common weeds that are particularly important as hosts for insect pests. Some of these include velvetleaf, lambsquarters, jimsonweed, purslane, pigweed, nightshade, and sunflower.
Not only do weeds harbor insect pests, but they also compete with your good plants for nutrients and water as well as for space in the garden.
Weeding may be one of the least desirable of all the garden chores but it is nevertheless very important. If you get the weeds while they are small and keep up with them over the season, the job will be easier and hopefully, you will be happily surprised at the reduction in the number of insect pests that call your garden home! This means you'll be spraying less pesticide in the garden - an added benefit!