Watering: When, Where, and How
From the Bountiful Gardens Archive

Water behaves differently in different soils and climates— so the more you know, the better you can decide how and when to do it. The time of day to water is controversial, with vociferous champions for both morning and evening watering. We like to take a more situation-specific approach. Watering will cool the soil, not just because the water is cooler, but through evaporation. So, in the spring, when nights are cold and it is important to let the soil warm up as much as possible, we like to water early in the day. That gives the soil all day to warm in the sun, and allows it to start the night as warm as possible.

Later, when the sun's heat is a threat as well as a boon, we might switch to evening watering, giving the plants the water they need to for growth during the night, when they are not heat-stressed. If the weather is brutally hot, we will water during the hottest part of the day. Some water will be lost to evaporation, but by evaporating, it forms as little pocket of cooler, moister air to refresh the plants. And it cools and moistens the tender roots.

We do not generally water the plants themselves, but the soil underneath. Water can be a cooling, cleansing, and refreshing shower for plants—they are used to rain, after all. But there are situations where it is best to keep water off of the leaves:

• When the sun is high and hot, water spots on the leaves—especially smooth leaves, like pepper plants—can act like a lens and actually cook the area underneath the droplets.
• If you live in an area affected by fungal diseases such as rust, or late blight of tomatoes, avoid wetting the leaves, which can spread fungal disease.
• If temperatures are right for powdery mildew, avoid evening watering; water in the mornings instead so the leaves have all day to dry.

We recommend mulch as well. Not only does it retain moisture in the soil, and add organic matter, it prevents disease. It works like this: studies (and our experience) show that fungal diseases, and even some pests, arrive on the plant with soil splashed up from the surface by water droplets. Even with a good watering wand, the drops of water from your hose have a lot of force: they compact the soil after constant impact, and they also cause drops to splash up on the bottom leaves. Commercial tomato growers sometimes remove the bottom leaves to avoid this. Mulch cushions the impact and holds the soil particles, so you get less compaction, less erosion, and less disease from splashback.

Your soil type makes a big difference in how water behaves in the ground. In clay soil, water takes a long time to penetrate, but spreads out over a large area, and stays moist for a long time. However, if it once dries out completely, it takes a long time to get completely remoistened. Pure clay will crack, allowing hot dry air to get at the root zone. If you find that cracks start in the unwatered paths and extend into the beds or your garden, you may need to water the paths as well, or at least mulch them.

Sandy soils allow the water to pass through quickly— so it goes straight down, and the top will dry out again soon. If you are using drip irrigation, you will need your emitters much closer together in sandy soils, and your watering times closer together as well. Hand-watering needs to be done often and sweep the entire bed.

Silt is a lesser-known soil type: it is very very fine sand, too fine to feel gritty. It takes a bit longer for water to run through it. But silt is very different than clay: The particles in clay soil are flat plates that interlock, and slide against one another when wet, almost feeling greasy. They are also held together by an electrical charge that holds water within the matrix of plates. Clay can be rolled into a “snake” or a bowl shape. Silt feels slick, but crumbles rather than holding a shape. In terms of watering, silt should be treated as a less-extreme form of sand.

Organic matter, like humus, compost, worm castings, manure, etc., will increase the soil's ability to hold water because it can absorb water within the particles as well as between them. It makes water available to plants gradually, mitigating both sogginess and dryness. The more organic matter you have, the less often you need to water, and the more available the water is to your plants. It make both clay and sand better for gardening. You can add to the effect of organic matter in the soil by mulching with organic matter on top of your beds. The mulch will increase biological activity in the top inch of soil, and allow feeder roots to flourish there without being killed by heat and dryness. (In cold, wet, soils, you should not mulch—but then you may not need to water much, either.)

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