Everyday Gardener: Mildewy basil, treated lumber, raised beds and more

This month’s column is a roundup of answers to problems or topics that gardeners have asked me about frequently in recent weeks. Some of these will probably be relevant to many of you, I hope. First up, gray, rotting basil: What gives?


Basil downy mildew

If you haven’t yet heard of basil downy mildew, you will. New to U.S. gardens in 2007, the disease has already been confirmed in more than a dozen states. The first sign of trouble is grayish spores on the underside of leaves. But these usually go unnoticed until the disease progresses and yellow splotches appear on the tops of leaves.

New basil varieties that are less susceptible to the disease will no doubt become more widely available over time, so look for those in the future. Until then, if you catch downy mildew early enough (when you spot the spores on the undersides of leaves), you can treat it with fungicides.

High humidity increases the likelihood of infection and we’ve had plenty of that recently. Avoid overhead watering as best you can, and do your best to provide good air circulation by keeping areas where basil is planted well weeded. Outbreaks in some states have been traced to basil purchased at big-box stores, so inspect plants carefully before you buy them. Better yet, buy your basil at farmers’ markets from local growers. This doesn’t guarantee you won’t have problems, but it probably reduces the risk.

Lawn watering

When it’s hot out and it hasn’t rained in awhile I see a lot of people out standing around spraying water on their lawns with the hose. I can see how this seems like a good idea, and lawns do respond by perking up a bit when they get a sip of water this way. But this is not a good way to water turf grass if you want it to be healthy and strong.

In the summer when it’s hot and dry lawns need about an inch of water every week. If they get that from rain, that’s great. If not, we have to do some supplemental watering ourselves and when we do it’s best to follow the mantra: Water deeply and infrequently. Let the sprinkler run until an inch of water (less if it has rained some) can be measured with a rain gauge, or in an empty tuna can or some other container.

Watering this way encourages a deep root system that will help keep grass thick and healthy, making it more capable of fending off weeds, pests and diseases. Frequently spraying the lawn with the hose, on the other hand, encourages a shallow root system. This makes grass more vulnerable to problems and far less drought tolerant. So resist the urge to spray and water well, instead.

Rotten strawberries

Talking with gardeners lately I’ve heard many times that strawberries aren’t doing so hot. Sadly, the combination of wet conditions and cool weather were perfect for gray mold. The fungus causes mushy brown spots that are often covered in gray spores that look like dust. It doesn’t take long for the entire strawberry to rot.

Because gray mold fungus lives from year to year in plant debris, clean up leaves and fruits that are diseased as soon as you see them and throw them away. Don’t compost them. Other ways to keep gray mold at bay next year include planting strawberries in areas with good sun, air circulation and drainage; pulling weeds to reduce humidity around plants; and picking fruit frequently. If your berries were finished bearing in June, mow or cut the beds back now and rake up and toss out any debris.

Treated lumber and raised beds

I’ve written about this before, so please forgive me if this is repetitive. It’s just that I’ve been asked about this issue a lot so it seems the answer bears repeating. As you probably know, there is much debate about the safety of using treated lumber around edibles, particularly when it is used to create raised beds.

In December 2003, the Environmental Protection Agency (EPA) banned wood treated with chromated copper arsenate (CCA) for home use over concerns about the possibility of arsenic leaching into the soil. In its place is ACQ, named for the compounds it’s treated with — alkaline copper quat. The lumber contains no arsenic that could leach into the soil, but studies have shown that some copper leaching does take place.

Copper is much less toxic than arsenic, but whether or not ACQ belongs near food crops is continually up for discussion. The only cautionary advice consistently offered by scientists and the EPA is that it should not be used near ponds and streams because copper is toxic to aquatic life. So whether or not to use ACQ-treated lumber for raised beds is a personal call, really. If you do, some suggest that lining the inside with plastic makes the issue of chemical leaching moot. Me? I would recommend using something else like cedar, bricks or stone.

Meleah Maynard is a writer and Master Gardener. Her Everyday Gardener column appears monthly in the Southwest Journal.