Dust is the enemy of plants in late August

The plants in our gardens aren’t looking their best right now. As the long, hot summer growing season nears its end in late August, plants clearly show the effects of heat stress, water stress and increased dust as well as air pollution.

Heat and water stress cause daily wilting, tip and leaf edge browning and slowed growth and flower production. Dust buildup on leaf surfaces blocks leaf breathing pores, stunting growth and killing leaves. When the air quality is unhealthy for sensitive people, a common occurrence during our summers, it’s also unhealthy for plants.

Air pollution, both high particulate levels and high ozone levels, causes leaf tip browning and leaf banding (brown perpendicular stripes on leaves), yellow stippling or spots on leaf surfaces, and leaf death on more susceptible plants and trees including azaleas, sycamores and birches.

As days and nights cool down a bit in late summer and a light breeze blows away some of the air pollution, perennial plants begin to recover. On roses, new leaf and bud growth is full-sized and plentiful; azalea and hydrangea leaves regain their normal dark green color; Japanese maples send out vigorous “whips” or long stems with vivid new leaves; wax begonias (which are often perennial in our climate) resume flowering, and new leaves replace the burned and browned summer leaves.

It’s the summer-blooming annuals and the summer vegetables that don’t recover well if they’ve been severely heat and water stressed, are covered with dust or suffering from the effects of poor air quality. Our fall planting season for cool-weather annuals and vegetables begins in mid-September and we need to prepare the soil for planting a couple of weeks before we set out transplants or seeds. We have to decide whether it’s worth the time and effort necessary to bring flowering annuals and summer vegetables back to good health for a few weeks.

Several types of summer annuals respond quickly to restorative measures. If we nurse vincas and impatiens back to good health-fertilizing lightly, cutting old growth back, washing off the dust and treating for red spider mites that thrive in dusty conditions, they’ll flower beautifully until the first frost. Tomatoes and squashes that stopped flowering during the hottest weeks begin to set flowers and fruit again in late August. The fruit should ripen before the first frost, usually in mid-November; row covers can extend the growing season of tomatoes, squashes, eggplants and peppers.

Beans, both pole and bush varieties, and cucumbers do not tolerate our intense summer heat well. Heat and water stress attract pest insects including whiteflies, mites and aphids. Spring-sown bean and cucumber plants should be pulled out now, but bean and cucumber seeds planted in late August can be ready for harvest in 50 to 60 days and they usually set a big crop. Buy seeds with the fewest “days to maturity” as shown on the seed packet label.

This time of year, give all your plants and bushes a thorough washing to remove dust and pest insect eggs. Monitor for continuing infestations of sucking insects and use insecticidal soaps to control populations. Feed monthly until mid-November with a low-number granular fertilizer or organic compost.

This story was originally published August 22, 2016 at 5:12 PM with the headline "Dust is the enemy of plants in late August."