It is a pleasure to answer your garden questions. Your curiosity is as refreshing and varied as the weather.
QYou recommend pruning forsythia by cutting it to 12 inches high and wide every year after flowering, adding that they lose ornamental value when sheared too often. What's too often to shear?
-- Charlie MaierAThe forsythia pruning guideline I gave assumes cutting it back once a year. Forsythia are often sheared three or four times a year to keep them in bounds by cutting off flower buds forming on new growth. I was suggesting that you cut the forsythia back hard (12 inches, or three feet if you want a hedge) immediately after flowers fade. Don't touch it again with pruners or shears until the following year, after it flowers. This method works only in full sun. Forsythia grows much more slowly and is more spindly in shade and might never need pruning.
When using vinegar for pest and weed control, should we use it straight or can it be diluted?
-- John Cece
Vinegar can be used right off the grocery store shelf. It comes as a 5 percent solution. Pickling vinegar, sometimes sold at Spanish or Asian grocery stores, is a 9 percent solution and twice as effective. Purchase it at garden centers labeled as a vinegar-based herbicide. This will provide you with instructions to follow. Apply it using a plastic sprayer; vinegar is highly corrosive to metal. And remember that it is non-selective and will burn any plants that it contacts. Don't spray on a windy day, and apply only to actively growing weeds. Spraying young weeds yields the best results.
We cut some pin oak in August 2006, split it in November and burned some last night. We had a hot fire of ash logs to which the oak was added. The oak burned slowly and left black soot on the back wall of the fireplace. Is this typical? -- Bob Vicic
I have heard that pin oak is undesirable firewood for its tendency to smolder. I disagree; it simply has to season long enough (nine to 12 months) before burning in the hearth. From your description of the way it smoldered, it hasn't seasoned long enough and might not be ready for several more months. Firewood seasons year-round and should be stacked and split to provide the best air circulation.
I have a mature American beech that needs pruning. I was advised to wait until it starts to leaf out so that we can determine which branches are dead. Will it be too late for pruning when new growth begins?
-- Irene Ewing
The beech will be okay if pruned in spring. However, as a rule, beech trees should not be pruned as they start to leaf out. A clean-out pruning this month would be perfect. If significant numbers of dead branches appear, have your arborist touch up the tree in summer.
Two years ago we planted a Japanese black pine in place of an overgrown yew. The tree is doing well, but we don't want it to outgrow the space as the previous tree did. Is there a way to prune it and maintain its size without spoiling the look of the tree? -- Charles Jensen
Other than bonsai training, the best practice for conifers is to not prune them, except for dead branches. Japanese black pine should be sited where it has room to mature. If pruned into a full, dense specimen, it will lose its handsome asymmetrical, windblown appearance.
If it's in good sunlight, planted at least eight feet away from structures and posing no encroachment problem, leave it alone. If not, transplant it to a better location and plant a smaller tree in its place, like Swiss stone pine or Japanese white pine.
We have some rhododendron plants that have consistently failed over nine years. We have a thriving birch tree at the corner of the house and wondered if this is robbing nutrients from the plants. It seems to be a problem isolated to a small area. Do you have any suggestions?
-- Lisa and David Jacobs
The rhododendron pictured in your e-mail died from a root rot disease, common to this region. It will always reside in your soil. There is no cure. Good cultural practices can help. Dig about one part leaf mold or other compost into your bed. Dig deeply to ensure good drainage. Plant shallowly, about one-third above the ground. Level off with compost and soil to the top of root balls. With good drainage, there is less chance of the fungus infecting your plants.
The following are plants with resistance to root rot diseases: mugho pine, globe arborvitae, blue holly, Sawara false cypress, Meyer juniper and sasanqua camellia.
When should I cut back my butterfly bush if it only blooms on new growth? -- Jane Rauen
What is meant when I write that butterfly-bush flowers on present year's growth is that the stems it grows this spring are the ones that form the flower buds opening this summer. Even if you cut it to the ground, your butterfly bush will grow back from its roots and flower this year.
If your shrub is growing in full sun, I recommend cutting it back one time to about 12 to 18 inches high and wide, every year before new growth begins. It will result in a fuller plant, which means more flowers and butterflies.
Cut fading flowers before they go to seed, a good way to deter them from becoming an invasive plant.
When is a good time to trim rosebushes? -- Florence Brodkey
Roses should be completely dormant before trimming them. This winter that never happened. I checked a rose garden on Presidents' Day, and there were still viable buds and green pliable leaves on the plants. If pruned, growth will be stimulated and open the plants to winter damage. Wait until the first week of March (weather permitting) before growth begins.
Does The Washington Post use vegetable-based inks, or, more generally, is The Post safe to use in a compost pile? I'm particularly worried about the color inks. -- Lisa A. Wainger
Yes, The Washington Post is printed with soy oil-based color inks and carbon black ink, and the paper is made primarily of pine, according to Washington Post spokesman Eric Grant. "It is 99.9 percent biodegradable." He added that soy inks have become pretty much a standard in the newspaper industry and could be used as a mulch that is safe in vegetable gardens and the compost pile. Here is how to use it for mulch:
· Clean up and cultivate the garden in spring.
· Spread open sheets of newspaper over the soil surface, three sheets thick.
· Cover with straw, shredded bark or compost.
The newspaper acts as a landscape "fabric" with moisture-holding and weed-control ability. Plant through it and cultivate into the soil at the end of the growing season.