I've received a cybersack of e-mails from you this summer, and I thought I'd answer some of them in this week's column.
Q: We have three crape myrtle trees heavily coated with mildew. Any suggestions?--Pat Nagel
A: A rather innocuous fungicide that helps kill mildew is Bordeaux, though it's best sprayed to prevent a problem rather than after you have one. You can also use a light horticultural oil or a fungicidal soap, being careful to follow label instructions.
Most fungi like cool, moist environments. But one anomaly of powdery mildew is that it instead loves hot weather, though I find it is more prevalent in shade or when there is poor air circulation. It can also be found on euonymus, dogwoods, lilacs, grapes, phlox and other plants.
Q: There is a vine growing up the wall of my garage. Originally, I assumed it was poison ivy. However, your article (June 12) suggested that poison ivy has three leaves on each stem; this plant has five leaves on each stem. Can you please identify?--Earl Johnson
A: You describe Virginia creeper (Parthenocissus quinquefolia). This native plant has compound foliage that bears five leaflets but otherwise resembles poison ivy. Take a piece to your local garden center to have this confirmed. It is a vigorously growing vine with great fall color. The good news is that there's no allergen in its sap.
Q: You wrote in a column (July 3), "When mowing, leave the grass clippings on the lawn. This helps hold moisture." My father always told me to rake the clippings up because it will choke the grass and hinder its growth. It always seemed to make sense to me.--Matt Zappone
A: Your dad was right, if you're mowing when your lawn is too long. You must rake clumps of grass clippings, or they can kill areas by blocking sunlight. But when mowing at the proper time, cutting no more than one-third of the blade height, the short clippings will fall down through the grass, decay and add organic material to the lawn within a couple of days. Grass decays quickly and is such a quick way to add organic material to the soil that it's sometimes called "green manure."
Q: I have two dogwood trees that I planted three years ago. I have been using gallon milk jugs, each with a pinhole in the bottom, to water them during the drought, the idea being to water them slowly but consistently with little loss to evaporation. Am I wasting my time trying to make this work? If it is a viable system, how far from the trunk of the trees should I place the jugs, and how many gallons per day should I be using? The system works well for my tomato and pepper plants, and I am trying to adapt it to other things.--Marianne Pontius
A: Clever system. The area that must get watered is the root zone, which is about as wide as the tree's branch spread. Water to a depth of seven to 10 inches. I'm not positive of the exact gallons per square foot for trees, but I would guess that it takes 60 gallons or more per 100 square feet. This slow-release method of watering is perfect to try, but make sure that enough water is released to soak deeply into the soil. There are also slow-drip bags called Tree Gators that are in great demand now. You would need only one of these bags to zip around your dogwood, and it would hold many more gallons of water than plastic jugs. You can get them at garden centers, if they still have some in stock.
Q: Thank you for the article on pruning lilacs--and how very timely of you. Now, what do I do with crape myrtle? One gardener says I can't prune at all; one book says it makes great cut flowers; and my garden has 20 of them, ranging from multi-trunked five-foot shrubs to 25-foot trees. I am concerned about controlling both height and width. Any advice will be greatly appreciated.--Ginna Dean
A: You really have an impressive-sounding crape myrtle collection. The only caveat is that the later you prune into the growing season, the more new growth you will stimulate, and with a harsh winter, new growth isn't as hardy as older woody stems. So it's best not to prune too hard after August for your least chance of winter kill. But flowers form on the present year's wood, so what you prune now will have no effect on next year's bloom.
You can certainly try crape myrtles as cut flowers; also, you can do light corrective pruning to develop the growth habit you want and cut dead wood out of them. They can be cut to grow as shrubs, or you can prune the lower branches and train them as trees. Crape myrtles whacked to the ground in spring can even come back as perennials, but don't shear them. Always prune selectively by cutting out the branches all the way to the trunk or where they join other stems.
Q: I have a whole lot of day lilies in my garden. Some of them have already reached their peak. Do I cut the spent ones down from the stem, or wait till the stems just wither away?--Peter Fernandes
A: The way to care for day lilies is to cut the scapes to the base right after flowering. That's about all they need. The best time to divide and transplant them is when they are done flowering, usually in July or August. Transplant with a lot of compost and water. For a more thorough discussion on transplanting day lilies and irises, read this column next Saturday.
Q: I have a viburnum (Chinese snowball type) that bloomed the first year I moved into this house (1994), but it has failed to provide flowers each spring thereafter. I mulch and fertilize each spring, and it appears healthy. The plant is probably quite old and is about five feet in height with an eight- to nine-foot spread. It needs a few branches pruned, which I plan to do next spring. Any suggestions or reasons why it is not blooming?--Kelly Jones
A: The main reason snowball viburnum doesn't bloom is that it's in too shady a site. Another reason can be too many nutrients. Stop fertilizing; mulch with compost about two inches thick, and try to get more sunlight to it by pruning dense tree limbs nearby. It should begin to flower again for you. Don't prune it until after you see if it blooms next spring.
Lerner is president of Environmental Design in Capitol View Park, Md. His e-mail address is email@example.com