By Scott Aker
Special to The Washington Post
Thursday, February 26, 2009
Q What are the correct practices for keeping potted hibiscus alive and healthy during the winter? I bought four last summer that are struggling.
A The tropical hibiscus is a large shrub with glossy leaves that requires full sun and warm weather to bloom well. Without a warm greenhouse, the aim in winter is simply to keep hibiscus alive until it can return outdoors. Find a room that is bright but also cool, which will allow a higher relative humidity. Expect most of the leaves to drop after you bring the plants indoors in the fall because of the reduced light and cooler temperatures.
Don't fertilize your hibiscus indoors. Look for mites and whiteflies on the leaves, and control them with an oil spray or insecticidal soap.
Your hibiscus will not grow well until night temperatures are warm, so wait until mid-May before putting them outdoors. Keep them in partial shade until new growth appears, then shift them to full sun. If your hibiscus plants are too big, you can prune them at that time.
Beginning in early June, feed with a liquid fertilizer every three or four weeks. Leave them in full sun and water them often enough to keep the soil moist. Cut back on feeding to encourage the production of more flowers if vegetative growth is rank and there are few flowers. Your hibiscus should bloom heavily in August and September and may be kept outdoors until early October.
I have many old azaleas that seem to be in decline: dead branches, smaller leaves, reduced bloom. They live in a large area of pines, dogwoods and camellias, with a solid ground cover of ivy. The ivy covers too much area to pull by hand. A local nurseryman suggested I mow the ivy as much as possible and then spray Roundup on the new growth in the spring. I've started the mowing process but wonder if it will be safe to spray Roundup around these shallow-rooted, stressed azaleas? Any other suggestions?
Glyphosate, the active ingredient in Roundup, is generally not harmful to plants unless it contacts the leaves, and the ivy leaves will shield the azalea roots. Ivy is not terribly sensitive to herbicides because of its waxy foliage, and repeat applications may be needed to get rid of it.
I prefer to kill ivy using a straight-edged spade, which must be sharp. Skim the ivy off the surface, severing the roots. I work in four-foot sections and roll up the mat of stems as I go. A preliminary mowing will help reduce the biomass you have to haul away. This is best done in the late fall or winter so the bits of chopped ivy will be likely to dry out before they have a chance to root.
Although it may seem impossible to tackle a space as large as yours, pick away at it and you will eventually get rid of it. Be sure to check cleared areas for any pieces you may have missed before they have a chance to grow large.
Scott Aker is a horticulturist at the U.S. National Arboretum.