To gardeners, Minnesota's weather always seems unpredictable. This spring, we've had a doozy. We started the season with a moisture deficit, which has only gotten worse for the southern half of the state, including the Twin Cities. Though perfect for picnics, the dry weather poses a major challenge for most of our garden and landscape plants. It affects everything from the smallest flowering annuals, which wilt easily, to our majestic shade trees, which are more prone to insect pests and diseases when stressed by drought.
When we have long dry spells, it's wise to set priorities for watering. First on the list should be young trees and shrubs that were planted in the past year or two. Water them thoroughly once or twice a week, depending on how hot it is. To help them retain moisture, mulch them with several inches of wood chips or shredded bark, starting an inch or so out from the trunk or stems and extending over the root area.
Newly seeded or sodded parts of the lawn are also a high priority for water. They require shallow, but frequent watering while their roots become established. Aim for twice daily at first, then gradually shift to deeper, less frequent watering as the roots grow stronger. Don't allow newly seeded or sodded grass to go dormant. It's not resilient enough to handle it.
Flowers and vegetables
More gardeners are growing their own vegetables this year, many for the first time. Vegetables need regular, deep watering not only for productivity, but to avoid misshapen or bitter-tasting produce. How often you water vegetables depends on the soil in which they're growing. In general, the sandier the soil, the more frequently you will have to water it. When you water, soak the soil to a depth of 6 to 8 inches. Don't wait to water until you see plants drooping. Repeated wilting can be very damaging.
Most flowering annuals need regular watering to maintain their good looks. Some - including moss rose (Portulaca), blue salvia (Salvia farinacea), flowering vinca (Catharanthus), gazania and California poppy - are more drought-tolerant than others.
On the other hand, perennials - especially those that are well-established - typically have deeper, more developed roots. That means you usually can wait longer between waterings.
Whether you're growing flowers or vegetables, adding a 2- to 3-inch layer of mulch will help conserve moisture, which will in turn keep the soil cooler and further reduce moisture lost to evaporation. When you're mulching vegetables or areas of the garden that are still developing, use straw, dried grass, chipped leaves or pine needles. You can use a more permanent mulch, such as wood chips, shredded bark or cocoa bean hulls, around perennials that won't be disturbed for a few years.
Before the early June rains, many lawns in the Twin Cities had already turned brown and crisp. It is possible to let your lawn go dormant, but not all grasses come out of dormancy well. Lawns containing common Kentucky bluegrass varieties withstand summer dormancy better than most newer "improved" bluegrass varieties. Also, while your grass may go dormant, many weeds don't. Tough weeds will continue to grow, making further inroads in your lawn.
Instead of risking dormancy, keep your lawn healthy by watering it regularly. When you water your lawn, you also water the feeder roots of nearby trees.
The best way to water is to soak the soil to a depth of about 6 inches (less if you have an automatic irrigation system and can run it every two or three days). Here's an easy test for when to water: If you walk on the lawn and your footprints in the grass don't spring back up, it's time to water.
When and how to water
It makes a difference when you water. It's most efficient to water early in the day, when temperatures are lower and winds are calmer. You can water at night, but plants will stay wet longer, which makes them more vulnerable to fungal and bacterial diseases. If you have sprinklers on an automatic timer, set them to start just before sunrise. Plants will dry rapidly once the sun comes up.
It doesn't hurt plants to water them in the heat of the day. In fact, it can help cool them. But it's not an efficient way to water because a large percentage of the water evaporates. So if your plants look droopy on hot afternoons, water lightly, then follow up with a thorough watering the next morning.
Try to water the soil rather than the plant itself. Soaker hoses and drip or trickle irrigation systems do a better job than sprinklers do in getting the water where it's most needed. Sprinklers, however, are far better than hand watering. It's difficult to deliver enough water to a lawn or garden when using a hose or a watering can.
Take care with container plants
In hot, windy weather, container plants can dry quickly. Water them in the morning when it's still relatively cool, and check them again in the late afternoon to see if they need to be watered again. Be sure to saturate the soil thoroughly by applying water until it runs through the containers' drain holes.
Frequent water can flush nutrients out of the soil, so you may need to fertilize more often. If you didn't use a potting soil with fertilizer in it or a slow-release fertilizer when you planted, add water-soluble fertilizer every two weeks if you're watering daily.
Deb Brown is a garden writer and former extension horticulturist with the University of Minnesota. To ask her a gardening question, call 612-673-7793 and leave a message. She will answer questions in this column only.