Garden Variety: Even in droughts, watering should be deep yet infrequent

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Plants wilt when they need water, but they also wilt when they have too much. The former is more likely in the Lawrence area right now, especially with recent high temperatures and the ongoing severe drought in much of northeast Kansas. But, despite the heat and drought, if you are dragging the garden hose out every day for your plants, you are probably watering too much.

The best way to water any kind of plant is deeply and infrequently. Think of the difference between a light rain that lasts all day versus a very heavy rain that lasts a few minutes. With the light rain, moisture has the opportunity to penetrate the soil surface and really soak into the ground, where it is taken up by plant roots. With a heavy rain, much of the water runs off the soil surface before it can be absorbed.

Deep, infrequent watering encourages deep root growth as plant roots search for soil moisture. Frequent, light watering provides moisture to the plant and discourages deep root growth. Then, if soil dries out more rapidly than usual, plants that have been frequently watered will show more stress than plants that have had a chance to grow stronger root systems.

Deep and infrequent watering is still situational. The best way to water is to always check soil moisture below the surface with a soil probe, long screwdriver, or other similar object prior to applying more water. The soil probe pulls up a core of soil that allows the best glimpse of the depth of soil moisture. A screwdriver or similar object will be difficult to push down into dry soil but should insert easily into soil with some moisture.

If soil has moisture below the soil surface, hold off on watering. Check again in a day or two. If soil is dry well below the soil surface, water plants that need it, including plants that lack drought tolerance, are shallow-rooted or pot-bound.

One way to experiment with depth of watering is to water by your usual method, then check moisture with one of these instruments. You might be surprised at how long it takes for the moisture applied at the soil surface to soak down into the soil profile.

Some plant species like more water than others, so some familiarity with individual water needs is helpful also. For example, hydrangeas will generally wilt at a moment’s notice in the summer, but deep and infrequent watering is still recommended. Infrequent could mean daily in this case, though.

On the other side, native plants and other drought tolerant species may not need supplemental irrigation even if soil is dry well below the soil surface. If in doubt, deeply and infrequently is still the best option.

What about tomatoes? They are probably one of the most common homegrown plants, even by those who consider themselves to be non-gardeners. Tomatoes are native to South America and are capable of producing very deep root systems. In the ground, avoid watering unless plants wilt. In pots, watering frequency depends on the size of the pot relative to the mature size of the plant. The smaller the pot, the more often it will need to be watered.

Mulching around the base of plants with plant materials also reduces the amount of water needed and frequency with which it should be applied. Wood chips, pine needles, bark, straw, prairie hay, and similar materials create an insulating layer that reduces evaporation of water from the soil surface. These mulches also reduce soil temperature fluctuations. Rock mulch can have the opposite effect and can reflect heat back onto the plant.

Plants in containers may still need to be watered daily or even twice a day if the pot is undersized for the plant or if most of the container is filled with roots. Watering deeply in a container means watering until water runs out the bottom of the pot.

Plants growing in raised beds, berms, or on slopes may also require more water than plants growing in flat ground (in the landscape, berms are mounds of dirt that are usually leveled out on top for planting). For soil types common in this area, infrequent watering of raised beds might mean once every 3-4 days. To water deeply, set up a soaker or drip irrigation hose, use a sprinkler, or let water trickle from the hose at the base of individual plants for an extended period (a few hours in most cases).

For most plants growing in the ground in most soil types common in this area, deep watering once a week is enough. Use drip or soaker hoses, sprinklers, or water individual plants with a low flow on the garden hose.

• Jennifer Smith is a former horticulture extension agent for K-State Research and Extension and horticulturist for Lawrence Parks and Recreation.


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