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The Honolulu Advertiser
Posted on: Friday, February 1, 2008

Chilly and wet winters tough on tropical lawns

By Jay Deputy

I often receive e-mail asking about lawn problems that occur more frequently at this time of year. Hawai'i's winter weather will often bring longer periods of cool, wet conditions that may lead to these special problems.

I have summarized a few of the most frequently asked questions and offer some suggestions.

Q. We live in Kane'ohe, and with all of the rain we've been having, we are getting a lot of mold/mildew on the ground. They are mostly in the bare spots where there is no grass. How do we get rid of them?

A. This growth is most likely algae rather than mold or mildew and is a common problem in parts of the yard where the soil stays wet. These areas usually have poor drainage and tend to accumulate water more than other areas. The condition is often worse when the area is also very shady. The algae are chlorophyll-containing micro-organisms that can form mats on the soil, sidewalks, patios or many other surfaces that remain moist for long periods of time. These algae mats range from light green to gray and take on a soft, fuzzy appearance when actively growing. As the surface dries out, these mats will form a hard, cracking crust, which greatly reduces water penetration into the soil. The result is an increased drainage problem, more standing water and a thicker algae mat during the next rainy period.

Algae mats in the lawn compete with grass and other ground cover, even weeds, and usually result in bare spots of compacted soil. They can become very slippery when actively growing on sidewalks or patios and become unsightly when dried out.

The best way to handle this problem is to prevent the cause. Improve drainage by removing the algae mat and loosening the soil. Mixing crushed cinder into the soil may help, particularly in low-lying areas. Most importantly, try to keep excessive water out of the area. Divert water runoff from gutter down spouts if that is the main source. A leaky sprinkler head may also be the cause. Patios and sidewalks can often stay too wet because of misdirected irrigation. If the problem area is in heavy shade, water will evaporate much more slowly. Pruning neighboring trees and shrubs to allow more sunlight penetration can help dry it faster.

Although the conditions that encourage algae mats are more likely to occur in winter, they are not uncommon during any part of the year. Overwatering in shaded low-lying areas can lead to algae mats even in summer and are a good indication of too much water. However, algae mats are not the only problems caused by shady wet soil conditions. Several types of weeds, most notably the sedge types, kyllinga and McCoy grass, are likely to invade lawns and ground covers in shady overwatered locations.

Q. I am getting spots of dead grass in my lawn. It has been getting more noticeable in the past few weeks during all the rain. Is there anything I can do to stop this?

A. Spots of dead or discolored grass can be the result of a variety of causes. Insect damage is one of the most common. In this case, the grass blades will be eaten away in everincreasing circular spots if caused by chewing insects like caterpillars, or the blades will remain intact but turn brown if attacked by sucking insects such as chinch bugs or soil-born grubs that eat away the roots. Close inspection of the damaged area will usually identify an insect pest and they can be controlled by a variety of insecticides. Insect damage is most likely to occur in warmer summer months in Hawai'i.

During cooler rainy-winter months, a few turf diseases may be the causal agent. They can be the result of fungal or bacterial organisms that need constant moist conditions to thrive. The symptoms of most diseases in turf are a gradual decline and discoloration in patches of grass, often accompanied by a slimy appearance. If a fungus is responsible, tiny threadlike fibers can be seen within the damaged patch.

These turf diseases are not particularly common in Hawai'i, but can become a temporary problem in wet, shaded areas of the lawn. A variety of fungicides and other chemical remedies are available, but unless the damage is severe, the condition will usually clear up as the weather gets drier and warmer. In the meantime, cut back on irrigation in the area, and when watering is necessary, do it in the early morning hours so the lawn has time to dry out before nightfall. Mowing can spread the disease, particularly when the lawn is wet. And as with the algae prevention, getting more sunshine into the area by some moderate pruning of trees and shrubs that are shading the area will help keep it drier and less susceptible to infection.

Jay Deputy is an education specialist in landscape horticulture and turf in the Department of Tropical Plant & Soil Sciences at the University of Hawai'i. Hawai'i Gardens, a column that rotates among a panel of specialists, appears here weekly.