Our situation can’t be compared with that of the western United States, but we are nonetheless experiencing a drought. April, May, and July were unusually dry, although we got a nice break in June, and temperatures have been higher than we’ve seen in a couple of years. Ridgewood Water has just imposed Stage II water restrictions, which means that homeowners who get their water from that company can water only two days per week (Tuesdays and Saturdays for properties with odd-numbered addresses, and Wednesdays and Sundays for properties with even-numbered addresses).
As a result of the drought, many plants are water stressed, and it’s easy to observe the different ways that various plant react. Large trees have such extensive root systems that they will suffer little permanent damage, but many have set seed unusually early (notice the heavy crops of maple seeds and acorns on the ground this early in the season). Many smaller trees and large shrubs will do fine as well, although they may show a little stress: leaves looking a bit droopy, a few leaves changing color early or dropping, smaller flower clusters. Perennials, if they are well established and properly sited, should be fine. The problem is plants that are not yet well established or that were sited incorrectly. Let’s consider each of those situations.
When you move a plant, it takes more time than you may realize to become established, or completely acclimated, in a new site. The bigger the plant, the more time it needs. A large perennial will probably need an entire growing season, a very small plant may be established within weeks, a shrub will take at least a year, a large tree will take several years. And for all that time, the plant needs supplemental watering during dry periods to help it produce new roots that reach far into the soil in its new home.
There’s a simple rule for determining how much water a plant needs. For the entire growing season, the plant needs at least 1 inch of water per week over its entire root system, either from precipitation or from supplemental watering. So if there’s half an inch of rain that week, give it half an inch more. If there’s 1 1/2 inches of rainfall, don’t water at all. If the weather is very hot, it can’t hurt to give it a total of 1 1/2 to 2 inches. In hot weather, more moisture evaporates from the soil.
How do you know how much precipitation you’ve received or how much water you’ve given? You place some sort of container–a cat food or tuna can, a yogurt container, any kind of container–on the ground among the plants, and you leave it there. After a rain or after watering, you measure the amount of water in the container. Once you’ve watered a few times, you’ll know how long it takes to deliver an inch of water with your sprinkler system or hose and sprinkler. I bet it’s longer than you expected.
Most people do not water deeply enough, especially those who use automatic sprinkler systems. Watering for 7 to 12 minutes per day, either every day or every other day, is very shallow watering. Shallow watering results in shallow root systems: the plants never develop deep, strong, healthy root systems that can help them withstand difficult conditions. Frequent shallow watering also encourages the growth of disease-causing fungus, especially water applied in the evening. On the other hand, deep watering just once a week encourages plants to develop deep root systems. Water deeply, and water early in the day.
Note that I’m concerned only with newly installed perennials, shrubs, and trees, not with established plants or lawns. Neither should ever require watering. Lawns naturally go dormant during hot, dry weather and green up as soon as it rains. Established plants have extensive root systems that allow them to mine the soil for every available molecule of water.
Established plants that wilt during dry weather to the point that they require regular watering present a different problem. These are plants that were planted in the wrong place: for example, shade plants placed in full sun or wetland plants placed in dry soil. These plants will need supplemental watering pretty much forever. To avoid this problem, do some research before you plant, and put the right plant in the right place.