Because of an early frost,in October, most of the trees and shrubs never lost their leaves. My questions:
1. Is this a problem? If so, what can I do?
2. A separate issue is when to cut back he stalks of perennial flowers. Does it depend on the type? What is the general rule, is there is one? His low should the stalks be cut? I do not cut back my shrubs until early spring. But I am less confident about hostas, cone flowers, sedum, etc.
1. The leaves on deciduous trees and shrubs eventually drop in response to shorter days and cool autumn temperatures. If there is an early frost, the leaves may be damaged by the frost, but this will not affect the health of the plants for the coming year. When the time comes for the leaves to fall, at the spots where the stems of the leaves meet the branches, cells called the abscission layer form and begin to cut off the leaves from the tree or shrub. The leaves will fall, then a protective layer will grow over the exposed areas.
2. Whether to cut back perennial flower stalks or grasses is an individual decision. Many perennials have seed heads that are attractive food sources for birds. The foliage can offer birds a good place to hide. By leaving perennials that are a bit less hardy “as is” over the winter, you may help them overwinter, as the foliage acts to insulate the crowns. As well, if you are like me and forget precisely where you’ve planted a beloved perennial that emerges later in the spring, by leaving the stalks over the winter, you are sure to find it in early spring! Finally, many gardeners love the aesthetic look of perennials left untouched over the winter – long stalks, dry foliage, stark outlines of former greenery, now covered in snow.
Penn State Extension’s Cutting Down Perennials in the Fall includes suggestions as to plants that benefit from being cut back over the winter, e.g., to minimize disease or pest problems. These include hostas (which you mention – the dead leaves could become home to slug eggs, which will produce critters that will feast on next year’s greenery!), bee balm (Monarda) and phlox (Phlox paniculata)
Mark Cullen’s Perennials: Some you cut down, some you leave standing also has helpful information – including “If it is ugly and you don’t like the look of it – cut it back”.
If you do decide to cut the plants back, do so after they have gone dormant (it is early December now, so if you have had a few hard frosts, now is a good time) to within 5-7.5 cm (2-3 inches) above the crown. Be careful not to cut too close to the ground, as the buds of some perennials are at or near the surface, as opposed to being under the soil line. And if you’re cutting back any diseased plant material, don’t compost it at home – check with your municipality as to how it should be disposed of.