Watch For Problems Created By Heat Stress; Complete Other Chores Like Dividing La. Irises

Daniel Gill, Merrill, Thomas A.  |  7/28/2005 1:46:30 AM

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Get It Growing News For 08/19/05

By Dan Gill
LSU AgCenter Horticulturist

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 this time of year for pests such as mealybugs, aphids, leaf hoppers, scales, spider mites and whiteflies. Monitor population levels and damage carefully. If an unacceptable amount of damage begins to occur, use an appropriate control method. Just be sure if you decide to use an insecticide to follow the label directions carefully and to use one that will be effective for the pests you have.

Shallow-rooted plants, such as azaleas, and trees and shrubs planted within the past year, also 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 also might exhibit stress symptoms. To help 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.

On the other hand, 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 them away from plants if extended periods of heavy rainfall occur. That will allow the soil to dry out.

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. Chlorothalonil (Daconil and other brands) will help control the disease if applied early and regularly, but the disease is not life threatening to the plant.

In August our yards and gardens may look a little tired. (After our long, hot summers, 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 chores include trimming back overgrown plants, especially bedding plants and tropicals such as lantanas, pentas, salvia, impatiens, periwinkles, hibiscuses and many others. We have such a long growing season that many of these plants have gotten leggy and less attractive.

Since they are currently blooming, trimming these plants 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 the late summer into the fall. It is too late to prune spring flowering trees and shrubs heavily, however, because they have already set flower buds for next year’s bloom.

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

Watering is commonly needed during dry spells. Irrigation generally is best done in the morning, but it is OK to water at other times if necessary.

Grooming plants by picking off faded flowers and unattractive foliage also is 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 depends on how close they were planted and how much room they have to spread.

Also, as clumps expand, they may begin to occupy more of the bed than was intended, and digging and dividing clumps is a way of maintaining it 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 replant the young rhizomes back into the same bed, another bed in your landscape or in pots to plant later or give away.

Before replanting, take the opportunity to improve the bed by digging a 2-inch to 4-inch layer of compost or other organic matter into the bed.

But be sure not to let the roots of the irises dug out of the bed dry out while you do this. Wet them down and cover them with a sheet 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. The top of the rhizome should barely show above the soil surface.

Then mulch the bed about 2 inches deep and water thoroughly.

This also is 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 still can select the colors you prefer from named cultivars.

Planted now, Louisiana irises 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 regular garden beds.

Get It Growing is a weekly feature on home lawn and garden topics prepared by experts in the LSU AgCenter. For more information on such topics, contact your parish LSU AgCenter Extension office or visit our Web site at A wide range of publications and a variety of other resources are available.


Contact: Dan Gill at (225) 578-2222 or
Editor: Tom Merrill at (225) 578-2263 or

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