Garden Detective: Eradicating crabgrass; growing Persian shield indoors

DEAR JESSICA: Over the last couple of years, this pesky, ground-crawling weed seems to be taking over my lawns. Can you please identify it and suggest ways to rid myself of it? — John Bruno, Lake Ronkonkoma

DEAR JOHN: That’s crabgrass. The quick-spreading weed thrives during summer and spends autumn making seeds, which drop to the soil and remain dormant throughout winter, even after frost kills the plant's top growth. In spring, the seeds awaken, sprout and take root, growing new crabgrass plants to repeat the cycle and further torment you.

The best way to eliminate it is to stop those seeds in their tracks — and timing is everything. The seeds will begin to germinate, or sprout and take root, during a very specific window of time when soil temperature reaches 55 degrees at 4 inches deep and remains there for at least two days. This is determined by the weather, not the calendar. Knowing when that is can be near impossible for us mere mortals, but Mother Nature provides clues for us.

Because the weather naturally affects other plants’ growth cycles, watch them for clues, and spring to action when you see them. You’ll do that by applying a pre-emergent weed-control product that will prevent the seeds from germinating — but you have to catch them just as they prepare to sprout. Don’t worry, it’s not as daunting as you might think.

The window of effective application begins when forsythia begins to bloom on your side of the street, and ends when the lilacs there fade. Don’t watch the plants across the street because the sunlight exposure is different from that on your side — and that makes a difference. If there aren’t any forsythia or lilacs on your side, observe the plants around the block at houses that face the same direction as yours.

The window typically is a couple weeks long, but when the lilacs fade, it has closed. Applications after that would be a waste of money because your crabgrass will have already sprouted.

For products, you have two options: synthetic pre-emergents, such as Preen Weed Preventer, or corn-gluten meal, a natural byproduct of the cornstarch manufacturing process. If you go with corn-gluten meal, look for the words “pre-emergent” on the label; there’s a livestock feed version of the product that isn’t effective against weeds. It’s sold under many brand names, as well as generically, and Preen packages corn-gluten meal under the label Preen Natural Weed Preventer, so read labels carefully.

Corn-gluten meal can be purchased in pellet or powder form. It's safe to use around pets and children, and around edible crops. Still, take care to avoid inhaling its dust or getting it into your eyes. Spread it according to package directions, typically 10 to 20 pounds per 1,000 square feet, then water the area once and let it dry. Timing application before a light rain will save you from watering, but avoid doing so before heavy rain, which may wash away the product. Generally, however, it remains effective for about six weeks. As a bonus, corn-gluten meal is high in nitrogen, so it can replace your spring lawn fertilizer.

Synthetic pre-emergents, such as Preen Weed Preventer, may be intended for use in garden beds rather than lawns; read labels carefully. Their active ingredient is Trifluralin, a chemical that can be toxic to honeybees, amphibians and aquatic wildlife. If used, it’s imperative to follow package instructions and dosing, and water afterward. The product absolutely should not be used near streams or ponds, or in areas where runoff into storm drains or bodies of water can occur. Use eye protection and gloves when handling. The chemical remains effective for roughly three months, but the increase over corn-gluten meal isn’t much of an advantage because of the limited germination window described earlier.

Both products prevent seed germination of all plants, not just crabgrass, so keep this in mind if you’re planning to seed your lawn or are relying on reseeding plants to self-propagate in the area. In this respect, Trifluralin would clearly be counterproductive.

Another chemical, Siduron, which is included in some weed-and-feed products, prevents crabgrass germination without affecting turfgrass seeds. Nevertheless, its registration and approved usage has been canceled by the U.S. Environmental Protection Agency, effective December 2020. That’s enough to keep me from using it, even though it will remain on the market through next season.

I’m team corn-gluten meal.

DEAR JESSICA: Can I keep Persian shield potted in the house? — Patti Carman, Port Jefferson Station

DEAR PATTI: Yes, you can! Persian shield is a striking plant grown for its shimmering purple leaves. It does flower, but if blooms aren’t removed, the plant will direct too much energy to developing them and likely grow spindly and leggy. It’s best to remove flowers, as well as side shoots, to prevent this and encourage a fuller habit.

Full to dappled indirect sunlight is preferred. If the plant’s color begins to fade, check to ensure it isn't exposed to direct sunlight; if it is, relocate it. Direct sunlight is more of a concern when the plant is growing outdoors, but it may present a problem in, say, a bright sunroom. Soil should be rich and well-draining, and kept consistently moist, but not soggy. Provide as much humidity as possible. This can be achieved by keeping the plant in a bathroom where it will be exposed to steam from daily showers, or by setting its pot on a layer of pebbles in a tray or saucer that’s kept filled with water. As the water evaporates, it will provide sufficient humidity around the plant (never allow the pot to sit in a saucer of water without pebbles, as that can lead to root rot.)