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Garden Q&A: When should I water?

Q: We’re seeing scattered rainfall this summer, as usual. Sometimes it downpours, but not for long. How do I know when to water?

A: It can certainly feel like a guessing game sometimes, since different plants have different water needs, and some neighborhoods get a surprise deluge mid-afternoon and others stay dry. Sometimes, heavy morning dew tricks us into thinking it rained overnight. Fortunately, the best approach is the simplest — stick a finger in the ground.

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Checking by feel is the most reliable way to determine what needs water before wilting starts. (You can sometimes trust wilting, but it’s not foolproof, and you want to avoid stressing a plant to that point anyway.) Judging the soil’s surface alone can be misleading and could make you accidentally under-water (if dew or a drizzle wet the surface) or over-water (if the deeper layers are still soggy). For shallow-rooted plants like annuals and certain shrubs, feeling soil moistness about a full finger’s depth (not including mulch) is a good test. For trees and shrubs, check 4-6 inches down instead. Sufficiently-moist soil will feel cooler to the touch, look darker, and stick to your skin. Soil which is getting dry will be warmer and dust off skin more easily.

When watering is needed, soak the soil for long enough that the water seeps down 8-12 inches (4-6 for annuals) to moisten the root zone. In most cases, infrequent but thorough soakings are much better for both the plants and water conservation than frequent, lighter waterings. If you use a programmable sprinkler system, check or adjust its settings, since usually their default is to run too often and too briefly. Not only does this waste some water to evaporation, it won’t encourage roots to seek deeper moisture, and the regular leaf wetness encourages disease. More information and tips are available on HGIC’s “Watering Trees and Shrubs” page.

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For plants in containers, the process is similar; smaller pots need checking more frequently, with the finger test at a shallower depth. Those in light plastic pots can be picked up to judge their weight — after a few checks you’ll get used to how light they feel when they’re getting too dry.

Q: What would cause my plants to wilt when I’ve been watering regularly?

A: There are many potential causes, but in essence, wilt happens when the plant’s vascular tissue (the transportation system which moves water upwards from the roots) cannot supply leaves with enough moisture. This could simply be due to dry soil, but it also could be due to damaged roots, or clogs or breaks in that tissue from fungi, bacteria, or stem-boring insects anywhere along that transport route.

You can’t do much about most of these causes after the fact except to trim out such damage once discovered, but you can be proactive about avoiding root rot by checking plants before assuming they need water. Over-watered plants can wilt because the oxygen-starved, ailing roots aren’t able to absorb that moisture (despite being surrounded by it). Heat-stressed plants can also wilt by day because they are evaporating moisture from their leaves to keep cool faster than their roots can resupply it. Hydrangeas and impatiens are notorious for this. If they look otherwise healthy and recover at night when the soil isn’t dry, this is a likely cause.

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University of Maryland Extension’s Home and Garden Information Center offers free gardening and pest information at extension.umd.edu/hgic. Click “Ask Maryland’s Gardening Experts” to send questions and photos.

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