For Release On Or After 08/27/10

Plants under heat stress are weakened, and we generally see an increase in disease and insect problems at the end of the summer. Keep your eye out for pests such as mealy bugs, aphids, leaf hoppers, scale, spider mites and whiteflies. Monitor population levels and damage carefully. If an unacceptable amount of damage begins to occur, use an appropriate control method. If you decide to use an insecticide, follow label directions carefully and make sure what you are using will be effective for the pest.

Shallow-rooted plants, such as azaleas, and trees and shrubs planted within the last year may be showing stress symptoms such as wilting, brown leaf edges and leaf drop. Trees whose roots have been damaged with lawn weed killers or by fill or construction might also exhibit stress symptoms. To help out these plants, make sure you mulch around young trees and shrubs with leaves, pine straw or bark. Also, water deeply and generously once or twice a week if a week passes without a good rain.

Diseases will be particularly bad if we get a lot of rain. Root rots are common in late summer and are best prevented by making sure beds are well drained. Mulches help retain soil moisture, which is generally a good thing, but it is a good idea to pull mulches away from plants to allow the soil to dry out if extended periods of heavy rainfall occur.

Cercospora leaf spot on crape myrtles is widespread when hot, humid weather also includes frequent afternoon showers. This disease causes the leaves to become spotted and then turn yellow or orange and fall off. The fungicide chlorothalonil – Daconil and other brands – will help control the disease if applied early and regularly, but the disease is not life-threatening to the tree.

In August our yards and gardens may look a little tired. And after our long, hot summer, gardeners look a little tired, too. Do most of your work in the cooler morning and evening hours, drink plenty of cold water and take frequent breaks.

Important late-summer chores include trimming back overgrown plants, especially bedding plants and tropicals such as lantanas, pentas, salvia, impatiens, periwinkles, hibiscuses and many others. With such a long growing season in Louisiana, many of these plants have gotten leggy and less attractive. Because they are currently blooming, this may take some courage. But be strong; cutting them back now will make them shorter, fuller and more attractive as they continue to bloom from late summer into fall. It is too late to heavily prune spring-flowering trees and shrubs, however, because they have already set flower buds for next year’s bloom.

Also, continue to keep up with weeding. Mulches are our best defense against weeds. Mulches you put down in spring may have decayed and thinned out by this time of year, and that makes them far less effective in weed control. Add more mulch now, if needed, to maintain a depth of about 2 inches for effective weed control.

Watering is commonly needed during dry spells. Irrigation is generally best done in the morning, but it is alright to water at other times if it is necessary. Grooming plants by picking off faded flowers and unattractive foliage is also important in late summer to keep our gardens looking their best.

Dividing Louisiana irises

If needed, Louisiana irises may be dug, divided and transplanted now through September. Each year Louisiana irises grow and spread, creating more rhizomes and shoots. Eventually, the plants may become crowded, which leads to lower vigor and poor flowering. How soon this happens primarily depends on how close together they were planted to begin with and how much room they have to spread. Also, as clumps expand, they may begin to occupy more of the bed than was intended, so digging and dividing a clump is a way of maintaining the bed at an appropriate size.

Louisiana irises are at their most dormant stage in the late summer, and that makes now the ideal time to divide or transplant them. To divide your irises, dig up a clump using a shovel or garden fork. Be careful not to damage the rhizomes. Break or cut off the young rhizomes – the ones that have new green growth at their tips – from the large, old rhizomes. Discard the old rhizomes and plant the young rhizomes back into the same bed, in another bed or in pots to plant later or give away.

Before replanting the irises, take the opportunity to improve the bed by digging a 2- to 4-inch layer of compost or other organic matter into it. Don’t let the iris roots you dug dry out while you do this. Wet them down and cover them with fabric to keep the roots moist. When you finish preparing the planting area, plant the rhizomes horizontally with the fan of foliage facing the direction you want the plant to grow, and carefully cover all of the roots with soil. The top of the rhizome should barely show above the soil surface. Mulch the bed about 2 inches deep and water it thoroughly.

This is also a good time to purchase and plant new Louisiana irises. Some local nurseries may have them for sale. And although they aren’t in bloom now, you can still select the colors you prefer from named varieties. Planted now, they will bloom far better this spring than those you purchase and plant in-bloom next year. Louisiana irises grow best in a sunny location and do well in aquatic gardens as well as in regular garden beds.

Rick Bogren