Free your plants of powdery mildew

Dear Jessica: I believe my 25-year-old peonies have powdery mildew. The stems are white and so are a few of the leaves. I don't see any insects. What is the best way to handle this? -- Rita Stasi, via email

Dear Rita: Powdery mildew is a fungus that presents as a white or grayish-white powder that coats leaves, stems and sometimes entire plants. Some plants, such as peonies, lilacs, phlox and hydrangea, are more susceptible to the disease than others, so they should be closely monitored. Stunting, leaf curl, yellowing or browning of foliage and premature flower drop are common symptoms of uncontrolled infections, but powdery mildew won't likely kill plants. Bloom times, however, are often shortened.

Affected leaves should be removed and disposed of (in the trash) as soon as they are noticed. Horticultural oil and Neem oil can be used to control outbreaks, and a synthetic, sulfur or biological fungicide may be needed for severe infections. Be sure to follow package instructions and precautions. If plants have been affected in past seasons, a preventive fungicide should be considered.

Good cultural practices can go a long way toward prevention. Simple measures such as planting in full sun, allowing adequate spacing of plants to accommodate their mature sizes, pruning to thin out branches or stems, dividing larger plants and removing fallen leaves and plant debris from garden beds can help keep plants healthy and prevent infections from spreading and reoccurring.

Dear Jessica: I read your recent column addressing ways to eliminate pokeweed. To get rid of the weed growing on a desirable bush, I cut it down to about a foot above ground level, use a sharp tool to remove the interior pulp and then pour some full-strength Roundup into the hollowed-out stem. This kills it every time. --Leo McSherry, Hempstead

Dear Leo: Broad-spectrum, nonselective herbicides like Roundup, which contains the active ingredient glyphosate, should be used with extreme caution, as they will kill whatever they come into contact with, including plants, trees, shrubs and grass. However, your method will help to eradicate pokeweed, as well as bamboo, which has hollow stems and so tunneling isn't necessary, and some woody plants, which must have holes drilled into trunks or branches before applications are made. When applied in this manner, the liquid will be drawn into the plant and extend to any suckers or offspring plants attached to the same root system. To maximize effectiveness, a second application is recommended 12 hours after the first.

The ends of cut branches or stems should be covered with aluminum foil after treatment to protect harmful exposure to wild birds and beneficial insects. And I cannot stress enough that precautions such as avoiding applications on windy days, covering nearby flora and pouring with precision should be taken as well.

SEE PHOTOSHow to overwinter tropical plants

Dear Jessica: Could you please tell me what the rule is about cutting back perennials in the fall? Someone told me you shouldn't do it until the first heavy frost. --Mike Angelastro, Sayville

Dear Mike: If you're referring to spring or summer-blooming perennials, like irises, black-eyed Susans and daylilies, then you can cut them down as soon as their foliage turns brown in late summer or fall. Perennials that bloom in autumn should be cut down in early spring. There's no rule for perennials about waiting until frost. There is such a rule, however, for some tropicals that must be cut down and then dug up in autumn and replanted in spring. They include canna, colocasia (elephant ears), caladium and banana.