Weeds and pest insects are often specific to a season. As the springtime weeds (crabgrass, poa annua or annual bluegrass) and springtime pest insects (hoplia beetles, snails and slugs) disappear from our gardens, we need to use different control methods to reduce summertime weed and bug populations.
Mowing at the correct height on crabgrass and annual bluegrass before the heads set seed is a good control method for those weeds, but prostrate and spotted spurges and purslane weeds grow so low that mowing is not an option.
Prostrate and spotted spurge seeds germinate when soil temperatures reach 60 degrees. As the weather warms, the tiny, purple-leaved spurge rosettes grow rapidly, forming a thick mat that chokes out other plantings. Spurges set seed in late summer and fall, but the seeds can remain dormant and viable for several years on the soil surface. Like all weeds, spurges are survivors and they thrive in thin, poorly irrigated, drought-stressed soils.
Cultivation of the soil surface with a wiggle hoe as soon as the tiny spurges appear will cut off roots. The long spurge tap roots may produce a second or third set of leaves, but with no leaves to feed the root, it eventually will die. A 3- to 4-inch layer of mulch will prevent spurge seeds from germinating, but will not prevent pre-existing weeds from growing through the mulch. Place weed barrier cloth or several layers of black and white newspaper underneath mulches if you already have a spurge problem. Apply a pre-emergent in early spring to prevent germination of spurge in early summer.
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Purslane is another low-growing weed that thrives in hot, dry weather and drought conditions. Its thick, fleshy stems and leaves hold moisture throughout the summer. Pull purslane plants as soon as they appear. We’re seeing many more purslane weeds taking over areas that were formerly lawns. Control for purslane also includes regular cultivation of the soil surface as well as mulching.
Caterpillars are a summertime pest insect. We see hornworms on tomato plants, red-humped caterpillars on redbud trees especially, and budworm holes in empty flower buds on petunias and geraniums.
Bt, or bacillus thuringiensis, is nontoxic to vertebrates but acts to stop the digestive system of all types of caterpillars, killing them quickly. Brand names for Bt include Safer’s “Caterpillar Killer” and Monterey’s “Bt.” Keep a spray bottle filled with a mix of the Bt concentrate and water handy to spray as soon as you see caterpillars or their damage or their droppings. Heavy infestations of caterpillars will need to be sprayed every seven to 10 days. Heat reduces the potency of Bt; replace the mix every couple of weeks during the hot months. Bt smells funny, so spray downwind.
As temperatures rise and new growth slows, snails and slugs stop feeding and find darker, cooler spots to hibernate. Spreading snail and slug bait in summer is not very effective. Look for sleeping snails underneath fence railings and north-facing window sills, inside overturned pots and on the undersides of old boards. If you’re still using overhead sprinklers for irrigation, you may see snails hiding underneath wet plants.
Send Elinor Teague plant questions at firstname.lastname@example.org or email@example.com (“plants” in the subject line).