New Zealand tea tree (Leptospermum scoparium)

I had a mimosa tree (Albizzia julibrissin) whose roots were invading a sewer line. I have taken the tree down to the stump. What is the best way of killing the roots? Is it better to remove the stump or drill holes in the stump and pour copper sulfate in the holes along with some water?
Tom Albert, Canyon Country
Once a tree has been cut down to the stump, you should no longer have to worry, in most cases, about its roots continuing to grow into sewage lines or anywhere else.
Roots grow from the energy they derive from a carbohydrate. This carbohydrate is manufactured in leaves and sent down to the stems through the trunk, and from there to the roots. Without a source of leaf-produced carbohydrate, roots simply will not grow.
Two notable exceptions to this rule are poplar (Populus species) and Brazilian pepper (Schinus terebinthifolius) trees. Even where stumps of these trees are dug up or ground to bits, residual roots many feet away will suddenly send up leafy shoots which, unchecked, eventually grow back into trees.
In the case of mimosa and many other trees, you may see suckers sprout from the stump after the tree has been cut down. However, as long as these stump suckers are cut or snapped off soon after they appear, they will not produce enough carbohydrate to benefit the roots and no more root growth will occur.
Some people drill holes in stumps into which chemicals are pored to eliminate suckering. This is really not necessary, as long as you keep removing the suckers by hand. Yet, if you do not want to be troubled with sucker removal, you could take the chemical route. Keep in mind that stumps whose suckers are consistently removed by hand, even without chemical treatment, will eventually stop suckering.
Tree trouble
Last year I was away for a week and my two New Zealand tea trees died, apparently from lack of water. My birch trees also seemed greatly distressed but this spring showed some signs of life long after other trees had leafed out. Now they, too, are dead despite deep root watering and fertilization.
At first I thought maybe it was something in my soil, but then I noticed my neighbor two houses down also has birch trees and they look dead also. I started looking around while driving home and noticed a few other similar-looking birches.
Is there something going around that’s killing the birch trees?
Sue Maki, North Hollywood
Your report on the death of New Zealand tea trees (Leptospermum scoparium) is not alarming since this species tends to be short-lived. While alive, they flower as brilliantly in red, white or pink as any other plant.
New Zealand tea tree is a misleading name since the species may grow into a 5- or 6-foot shrub, but never a tree. It has a relatively brief annual flowering period and, more often than not, is kept trimmed back because of its terribly drab appearance when not in bloom.
We have just experienced the 12th-hottest summer in Los Angeles since weather data began to be logged more than 100 years ago. This fact could help to explain the premature demise of white birch trees, indigenous to colder and wetter climates than our own.
Although they are not long-lived in any case, unusually hot weather will contribute to the early decline of white birch trees.
Prune white birch trees lightly, if at all, since they are easily invaded by deadly fungi when large cuts are made in their branches.
Plant mildew
I have two hydrangea plants that are growing and flowering very nicely in a filtered light to shady exposure. However, one of them is covered with mildew.
Sometimes in the afternoon, if they are droopy, I water them. I suspect this afternoon watering is causing the mildew. No matter what I spray them with, the mildew is not going away. In fact, it is getting worse.
Should I just wait until December, cut each plant back for the winter, and be sure that I do not water them in the afternoon when they begin growing again next spring?
Lois Drever, Simi Valley
The powdery mildew fungus you describe proliferates during warm weather and, as you suggest, cutting the plants back in winter is a good idea. However, I would wait until late February to do so, since cutting back in December could stimulate new growth that would be killed during a frosty winter night.
Keep in mind that initial powdery mildew infections often occur in the morning, typically as a result of fungus (mildew) spores germinating in droplets of dew that settle on the leaves. Unlike other fungi, powdery mildew is not moisture-dependent and can grow under dry conditions.
Among rose growers, it is a common practice to hose down rose foliage early in the morning to knock off mildew spores that may have settled there. Starting next spring, once new growth has begun, this same procedure might be helpful with your hydrangeas.
Tip of the week
To combat powdery mildew without resorting to toxic chemicals, mix one teaspoon of baking soda, one quart of water and a few drops of dishwashing liquid in a spray bottle.
Apply to foliage either to prevent mildew growth or immediately after mildew appears.
A mild horticultural oil such as Sunspray Ultra-Fine Spray Oil is also recommended for control of powdery mildew.