Q On my travels I have noticed two different trees that I would love to grow, but I am not sure they would survive in the Washington climate. One is the mountain ash, the other a eucalyptus. Would they work here?
A There is only one mountain ash that I recommend for our area -- Korean mountain ash, Sorbus alnifolia. It is the most heat resistant of the ashes, and can tolerate our summers. American mountain ash is a northern species and succumbs quickly to the heat of our summers and a host of pests and diseases.
Eucalyptus presents a different quandary. There are species that are superbly adapted to hot summers like ours, but they are not winter hardy at all. There are also species native to the Southern Alps mountain range in Australia. They are quite cold hardy but cannot tolerate heat. If you want interesting bark, try growing a stewartia, a paperbark maple, or a crape myrtle.
My oakleaf hydrangeas have gotten huge. When and how do I prune?
Prune them by removing entirely some of the thickest, woodiest stems every few years. This will reduce the overall mass of the shrub. The remaining younger stems have more vigor and tend to produce the showiest flower clusters.
Like most hydrangeas, oakleaf hydrangea is a shrub that sprouts new growth from the base of the stems on a regular basis. Even if cut back severely, it will recover quickly. It doesn't grow as vigorously as quince or forsythia, so it probably won't need to be pruned more often than every four or five years.
Late winter would be a good time to prune because the branches are more easy to discern when the large leaves are no longer present. Don't use this advice for all hydrangeas. Florist hydrangeas -- the popular mopheads and lacecaps -- should never be pruned in our area until after new growth is evident in the spring. Winter dieback is often a factor in these somewhat tender varieties and it is nearly impossible to tell what parts of the plants have survived until the new growth appears.
In the fall, my small Japanese maple was flooded about four times in one month. It is next to a small pond. Before the natural leaf fall of autumn, about half the leaves turned brown and turned dry and brittle. Have I lost my tree?
Wait until the spring to see what is alive and what is dead before you perform any surgery on your maple. Dead leaves aren't really a good indicator of dead branches, particularly at the end of the growing season. I have a feeling that the prognosis for your maple is good. Maples can tolerate a short period of submersion better than some trees, and flooding is less damaging in the late summer and early autumn when active growth is not taking place. Flooding in spring is far more harmful because the roots are very active and have a high daily demand for oxygen as growth begins.
However, young trees and shrubs are easily moved, and you may want to relocate your maple to higher ground. Do this in late winter while the plant is still dormant. Be mindful, though, that Japanese maples do not thrive in exposed, hot sites with poor soil. With far-off summer in mind, give your maple a little afternoon shade, good soil and a light mulch for the root zone.
Scott Aker is a horticulturist at the U.S. National Arboretum.
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