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LifestyleColumnistsJessica Damiano

Garden Detective: Perennials with black spots, blooming snake plant

Spots indicative of a fungal or bacterial infection

Spots indicative of a fungal or bacterial infection plague plants in reader Dorothy Cesarski's garden in Sayville. Credit: Dorothy Cesarski

DEAR JESSICA: All my perennials have black spots or holes in their leaves. It happened last year, too. I thought it was from the mulch I put down, so this year I used peat moss instead. If my soil has bugs doing this, is there any hope? Or do I have to dig this up and start over? — Dorothy Cesarski, Sayville

DEAR DOROTHY: Black or brown spots on leaves usually are caused by a fungal or bacterial infection. Either way, the best defense is a good offense: good sanitation.

That means vigilantly clearing away (and disposing of) fallen leaves and plant debris, and removing infected leaves from the plant when you first spot them so they don't infect the rest of the plant or nearby plants.

And although you might ordinarily leave the plant's aboveground growth in the garden until spring cleanup, you should cut infected plants down at the soil line now, and dispose of them properly (tightly bagged in the trash). This step will help prevent pathogens from overwintering in plant parts and reinfecting the garden next year. After trimming down plants, be sure to sterilize pruners and other tools used with either a 90-10 water-bleach solution or with a drenching of disinfectant spray.

Air circulation around and within plants is important because tightly packed foliage retains moisture and blocks sunlight to plant parts, both of which create the perfect breeding ground for bacteria and fungi. So next year, be sure to monitor plants for crowding, and strategically prune stems and branches as needed. Finally, avoid overhead watering, which wets foliage. Instead, direct the water to where it's needed — the roots, either with a handheld hose or soaker hose snaked on the soil around plants.

The holes in your plants’ leaves are typical of the uneven bite marks left by slugs. The slimy visitors aren’t terribly finicky but hostas are among their favorite menu items. And they prefer moist, dark and cool places, such as under those giant hosta leaves. You’re not likely to spot them, either, because they’re nocturnal, emerging to feed only after dark. But the damage they cause can be devastating.

Good cultural practices, like opening up air circulation around plants, helps make conditions unfavorable for them, and because slugs also consume decaying matter, it’s best to keep mulch around plants to an inch or so deep.

One way to trap them is to lay 1-square-foot pieces of wood, carpet remnant or heavy cardboard in areas where you see activity and leave it in place for a couple of days. Lift the square in the morning, and you’ll likely find them hiding beneath it. Scrape them off and destroy them. Another effective trapping method is to fill shallow containers with beer and sink them into the soil, leaving one-half inch at the top exposed. The beer will lure the slugs, and they will drown.

If all else fails, use a product containing iron-phosphate, such as Sluggo, Escar-Go! or Garden Safe Slug & Snail Bait, all of which are safe to use around edibles, children and pets, and do not harm other wildlife.

To address your mulch concern: Applying it too deeply certainly can encourage pests and diseases; keep it at 3 inches deep or less. But switching to peat moss was a mistake.

Peat moss is highly acidic, so using it around non-acid-loving plants can stress or even kill them. It’s also lightweight, so can be blown away in windy conditions. Used as a straight mulch, peat moss hardens when wet, and can form a barrier that prevents water from reaching plant roots. Then when it dries, it cracks. In addition, peat moss isn’t a sustainable resource.

Peat moss can be useful as a component of homemade (or commercially packaged) potting mix because of its light weight. And although it contains no nutrients of its own, combining (and fully incorporating) peat moss with compost, vermiculite and perlite can aid soil nutrient availability and makes a wonderful seed-starting mix. This concoction also can be applied around acid-loving plants, such as rhododendrons and azaleas, and can even be used to lower the pH around hydrangeas to turn pink blooms blue.

DEAR JESSICA: My 6-year-old snake plant has suddenly sprouted a stalk with clusters of yellow-white flowers. This plant was started with cuttings from a much older plant that grew too large and cumbersome to keep. It has spent the summer outdoors under a covered deck with limited access to sun and rain. The flowers are attracting ants. I’ve never seen anything like it before. — Pat Visconti, Islip Terrace

DEAR PAT: It's quite rare for snake plants (Sansevieria trifasciata) to bloom, but when they do, as you've noticed, they send up a long stalk that becomes covered with strongly scented, cream-colored flowers. It’s the sweet scent that’s attracting insects, and the show you’re witnessing is an indication your plant is stressed.

My guess is the plant is probably root-bound and would benefit from being moved to a container that’s 2 inches larger in diameter (ensure it has drainage holes at the bottom).

When the flowers stop blooming, remove the plant from its pot and gently tease apart its roots to encourage them to grow outward instead of encircling and choking themselves.

Fill the new pot partially with a potting mix intended for tropical houseplants and sit the plant at the same depth as it was previously, then pack more of the potting mix around and over roots, and water well. Keep the plant out of direct sunlight for a couple of weeks while it settles in, and don’t fertilize until spring. This should be done every year or two, but never into a container that’s more than 2 inches larger.

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