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How to Water Your Garden
Deep Watering Is a No-No
Think of the moisture in your garden soil like a half full glass of water. Each day some of the water is lost through transpiration and evaporation, and each day you must replenish what is lost. The best time of day to water is early morning, providing the soil with a reservoir of moisture for plants to draw on during the heat of the day. Even with adequate water, some plant leaves are flaccid during the hottest part of the day because they transpire water from the leaf surface faster than they can absorb water through their roots. This is most visible during hot midsummer days on large leaved plants such as pumpkins and squash. If it is an exceptionally hot day, you may also need to water late in the day, after the heat has begun to subside. If you see that the leaves on your plants have not returned to a turgid state when temperatures begin to cool, they need water.
The rate at which water is lost through evaporation and transpiration is called the evaporation rate. For the scientifically inclined, this is a specific measurement that takes into account air temperature, cloud cover, wind and other factors that contribute to terrestrial water loss. For the rest of us, the significance is that the soil loses water at different rates each day, and you need to be aware of the aforementioned factors when you water your garden. The easiest way to find out if your garden has enough water is to occasionally dig 6-8" deep and check the soil moisture. If it is like a wrung out sponge, it's fine. If it's bone dry, your plants need water. If the soil is saturated, which it may be after a heavy rain, hold off on watering. Above all, do not base any judgments on the top two inches of soil. They are not valid because of the surface soil's exposure to the air. You need to know what's going on where the plant roots grow.
Little Plants Need Water. Big Plants Need More
If you are starting plants from seed, it is essential to water at least once each day, preferably twice on hot days, to ensure that your seeds germinate. The most common cause of seed failure is drying out.
As the days grow longer and hotter and your plants increase in size, they need ever more water. This is critical for fruit production of our favorite summer vegetables. To illustrate, in our trial garden we use drip irrigation to water our plants. In the relative cool of the late spring our irrigation system is set to two cycles per day of approximately ten minutes per cycle. By August, when the plants are large and productive, the morning cycle is as long as 45 minutes and the afternoon cycle is at least 15 minutes. We begin to scale back on the duration when the days grow noticeably shorter and the nights cooler, usually late August.