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Plants behaving badly? Rethink your technique

Friday - 7/27/2012, 12:15pm  ET

Sold under brand names like Dipel, Thuracide and Green Step, Bt only kills caterpillars that chew on the sprayed leaves. The ones that become butterflies will be fine. (Thinkstock)

Don't over-water your lawn

Mike McGrath, Garden Plot


Mike McGrath,

Evening watering means dead squash and cucumbers

Hastings in Woodbridge writes: "My squash and cucumber plants have developed a white mold on the leaves. It's so bad that my squash plants have all died. What causes this and is there anything I can do to save my cucumber plants?"

I emailed Hastings back with a standard set of questions about food, watering and mulching.

"I only feed them Miracle Grow, they're mulched with wood chips and I water in the evenings, sprinkling the plants for about 20 minutes," he replied.

Well then, you're the cause of the powdery mildew that's killing your plants, Hastings!

Chemical fertilizers like Miracle Grow weaken plants, especially when the plants also have to battle problems like this summer's extreme heat.

That crappy wood mulch is a perfect disease incubator and has helped your mildew spores breed in abundance.

And wetting the plants' leaves in the evening was the knockout punch. No plant likes having wet leaves overnight, but it's especially deadly for broadleaf crops like squash and cukes. You should only water plants in the morning and you should always try to avoid wetting the leaves. When you water, water deeply once or twice a week. Short, daily waterings are plant killers.

Pull off and destroy all the affected leaves. Don't leave them in the garden or try and compost them. Stop with the chemical plant food - replace the wood mulch with compost. Switch to morning watering - only at the root zone - and the new growth will be fine.

Bitter cucumbers? Oh, bitter day!

Chris in McLean writes: "Why are our cucumbers so bitter? Lack of water? Do we let them get too large? We grow in raised beds, don't feed them, they're mulched lightly with straw and are watered 'regularly' with a hose."

Variety choice is important, Chris - some cukes are bred to be nice and sweet while other varieties sacrifice flavor for traits like disease resistance and heat tolerance. So next year, make sure you pick a variety touted as sweet and flavorful.

But starvation will also make crops like cucumbers and lettuce taste bitter, so give the plants a good organic feeding with something like worm castings or a liquid fish and seaweed fertilizer. Don't feed them chemical plant food. It will make the bitterness worse.

And "watered 'regularly' with a hose" doesn't tell me anything. But you could well be watering them incorrectly. For the best flavor, soak the bed deeply once or twice a week. Short daily sprinkles equals bitter taste.

And do start picking them on the small side. Unlike tomatoes and peppers, cukes and summer squash are edible as soon as they're visible. And, in general, the smaller the fruit, the sweeter it'll be. Overripe cukes - ones with full sized, viable seeds inside - will always develop a bitter taste. And pick them early in the morning after the cool nighttime air has concentrated their sweetness. Cukes picked in the heat of the day or in the very early evening will always taste less sweet.

Bt kills pest caterpillars, not butterflies

Dallas in Arlington writes: "My petunias and geraniums are under attack from a pest I've identified as budworm. I've been picking them off manually, but I'm losing that battle as I have almost no flowers right now. I've read about Bt as an organic treatment, but apparently it kills regular (butterfly) caterpillars as well. Can you please advise me?"

My pleasure, Dallas. Get some of that organic caterpillar killer known as Bt and spray away! Sold under brand names like Dipel, Thuracide and Green Step, Bt only kills caterpillars that chew on the sprayed leaves. The caterpillars that become butterflies are not the ones eating your flowers. The little buggers you're battling will grow up to be ugly moths. Bt is so safe and pest specific that an adult butterfly could land on the sprayed plants and not be harmed.

In general, the only garden plants attacked by caterpillars that will become beautiful butterflies are dill, parsley and carrots, so handpick any worms off those plants or just grow extras and enjoy the swallowtails that follow.

Trumpet vine terror

Debbie in Damascus writes: "A few years ago I planted trumpet vines on a trellis over my fish pond to provide shade and limit algae growth over the summer, but now I have trumpet vine growing everywhere: in the grass, up through the deck....EVERYWHERE! I say just pull it up as it appears, but my husband is crazed about it! Help!"

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