Garden Docs: Advice for Sonoma County on getting more blooms from crape myrtle trees

Robert K. asks: Why haven’t my crape myrtle trees bloomed much this year? There are three trees, approximately 12 feet tall, planted in a row on the east side of my house and have developed a height and width over time that makes for the perfect privacy screen as well as providing summer color, bark texture appeal and fall foliage color. They have not been thinned or pruned for the last two years. I have read that it isn’t always necessary to prune crape myrtles, but now I am questioning that advice. What are your thoughts?

First of all, thank you for providing a visual picture of their location in the garden, including sun exposure, height and width.

Crape myrtles bloom on current year’s wood and should be pruned back hard each year after the bloom. Yes, we know that we see many crape myrtles used in the landscape as street trees, not pruned every year, and they are blooming happily. But they are in full sun, not creating their own shade, and are probably pruned more often than we think.

Here is our advice: Prune (head back) your trees by a least a foot. Strive for an open vase shape to allow light to enter the canopy. During the process, thin out all twiggy growth, old spent flower clusters and unwanted sucker growth. This type of pruning will stimulate new growth and promote a full bloom. As an added thought, it wouldn’t hurt to give your trees an all-purpose fertilizer. Follow the directions for the exact amount for their size. Water the soil first, incorporate the fertilizer at the foliage drip line and water the fertilizer in thoroughly. It seems, in your situation, more frequent pruning is a must to promote a nice yearly flush of bloom.

Ruthie asks: My weeping cherry, Prunus pendula, was planted in 1992 and over the years it has given me unending joy. Now I am very concerned about its health because it has developed a shiny black, hard “bead” at the graft. From this bead there extends a light-colored stain about 1 inch long and quite narrow in width. When I touched the “bead” and stain, neither was sticky. What do you think caused this problem and do I need to worry? Amazingly, it has always appeared healthy and grows in soil that has good drainage.

Oozing sap is called gummosis, Cytospora canker, and is caused by one or more of the following reasons:

Borer damage. Look for small holes in the bark.

Natural tendency. Small beads often appear on the bark of healthy trees.

Disease. This is often the case in newly planted trees that are at particular risk of infection if replanted in areas where the disease previously occurred.

Mechanical injury. Often caused by a lawn mower hitting the bark, weed whacker, garden pruners used incorrectly or incorrect staking used on young trees.

Sunscald injury. More often seen in young trees not protected from scalding, it happens when direct sun on the tree trunk causes open cracks and wounds. Painting tree trunks with a diluted white latex paint that will reflect the scalding sun rays can prevent this type of injury.

If there is evidence of injury, apply a protectant fungicide, labeled for use on Cytospora, as soon as possible. If you are unsure about the product, check with Purity Chemicals and they will give you expert advice.

Borers are extremely difficult to control. If you do see a small hole where the sap is oozing out, use a thin wire to probe in the hole and kill any existing larvae.

Lastly, you describe your beautiful weeping cherry as always appearing healthy, and perhaps the gummosis is simply a natural tendency displayed by the tree.

Eric A. asks: I have a coleus that has pretty light blue flowers. Should I cut them off? Also, the plant is growing tall, and is looking more tree-like instead of full with masses of colorful foliage. Otherwise, it appears healthy. Any advice or tips would be appreciated.

If you cut off the flowers it will encourage the development of more side shoots, thus creating a fuller appearing plant. Also, pinch back many of the lanky stems directly above existing leaves.

It may seem a little bare at first, but it won’t take long for new side shoots to appear with fresh colorful foliage.

I have found that emptying my unfinished cold coffee onto the coleus’ container soil is the perfect daily fertilizer.

Friends have asked me for my secret to growing beautiful coleus and that is the simple answer. The coffee has acid and coleus prefer an acidic type fertilizer. You might give it a try.

Dana Lozano and Gwen Kilchherr are garden consultants. Send your gardening questions to The Garden Doctors, at pdgardendoctor@gmail.com. The Garden Doctors can answer questions only through their column, which appears twice a month in the newspaper and online at pressdemocrat.com.