Garden columnist Dan Gill answers readers' questions each week. To send a question, email Gill at [email protected].
I’m having an odd problem with my Mexican petunia plants. Nothing seems to bother them, but this summer, I’ve noticed white splotches and spots on the foliage. The plant is continuing to grow, but the foliage does not look good. I’ve looked for insects and don’t see anything. Is this some sort of disease? Is there something I can do for it? — Harry
This is a relatively new problem for Mexican petunias (Ruellia) in our area. At first glance, the white spots could appear to be a fungal issue or even a chemical or paint residue, but it’s not. These white splotches found on ruellia leaves are a type of growth distortion called an erineum, and they are caused by tiny mites called eriophyid mites (Acalitus ruelliae).
The mites are not life-threatening. In most cases, the population will decline, and the plant will continue to grow and bloom without issues — even if not treated to control the mites.
If you decide to control the mites to improve the appearance of the new growth, you can treat the plants with a light horticultural oil like All-Seasons Oil Spray or Year Round Spray Oil. Repeat the treatment every five to seven days for a total of three treatments. After treatment, new growth should emerge healthy.
Another approach would be to use spinosad (various brands) to treat the mites. Make two or three applications following label directions. I doubt treatment is ever critically needed, although the white spots are unsightly.
I need some information about fertilizing a newly laid lawn. Should I fertilize the ground before I put down the sods, or should I wait until after? Also, what is the best fertilizer to use? — Mary
It's best not to fertilize newly laid sod before or immediately after it is laid. The sod loses most of its root system when it is harvested so there are few roots to absorb fertilizer. And fertilizer can be detrimental to the sod as it establishes. Not having roots makes it vulnerable to drought, and fertilizer compounds drought issues. So, don't worry about fertilizing the sod initially.
Watering is what is most critical. A good schedule is to water every day with a sprinkler for about 20 minutes for seven to 10 days. Then, water every other day for about 30 minutes for another seven to 10 days. After that, water as you typically would. Water thoroughly once a week if the weather is dry.
After about six weeks, the sod will have grown a root system, and you can fertilize. A Turf Builder would be fine for an establishing lawn. But you could use any fertilizer other than weed and feeds products. Don't use those.
It's getting late to trim leggy azaleas. Remember: The best time for hard trimming is in the spring.
I have azaleas that have grown a little tall. They have generous leaves on top but appear leggy lower down. How far can they be trimmed down so they will grow fuller and bushier? Thank you for your assistance in this matter. — Francis
It is getting late to trim azaleas. This should be done within the next week at the latest (it’s too late to prune Encore azaleas as they will begin to bloom soon; they should only be pruned in spring when they finish blooming).
To stimulate growth in the lower parts of your azaleas, you need to cut them back to the point where you want the lower growth. New growth will be stimulated from the location where the cuts were made. Keep this in mind when deciding how far back to cut the plants.
It's generally fine to cut an azalea back about one-third to one-half its height. Hopefully, cutting it back will do the trick.
The best time to do hard pruning like this is in the spring when the azaleas finish blooming and begin their main growing period. Pruning severely during the extreme heat of midsummer is stressful to the plants. So, this is not the best time to do that type of pruning.
Garden tips
HERB TLC: Remember to harvest herbs such as mints, basil, rosemary, lemon balm and Mexican tarragon regularly to keep the plants shapely and under control. Some herbs such as thyme, sage and lavender tolerate heat and rain poorly and may not be doing well now as a result.
A LITTLE TRIM: A long growing season and rapid growth often lead to overgrown beds this time of year. Feel free to trim bedding plants and tropicals to keep them under control. Stake or otherwise support plants that need it.
WEB SITES: Webworms, caterpillars that form tents of webbing at the ends of the branches of various trees (especially pecans), look bad but rarely do much damage. If control is needed, spray with a product containing BT (Dipel, Thuricide), spinosad (various brands) or other labeled insecticides. Make sure the caterpillars are still present in the webs before you spray. Although webworms look terrible, they will not significantly damage the trees and shrubs they infest. They will recover even if you do not control the webworms.
WATERING: Overall, the summer so far has been somewhat dry, but rainier weather is bound to occur. Be sure not to irrigate unless the weather and soil are dry enough to warrant it. Irrigating when not needed often creates root problems, such as root rot. On the other hand, hot weather can quickly dry out landscapes and lead to stress. Pay attention to irrigation — don’t water excessively, but don’t allow your landscape to get too dry either. One or two rain gauges placed where you can easily see them will help you determine how much rainfall your landscape is receiving, and this can help tremendously when determining if irrigation is needed.