By Andrew Padula, Contributing Columnist and owner of Greer-based Padula's Plants:
The best time to water your garden is early in the morning or later in the evening. Watering your plants in the morning allows time for the soil around your plants to soak up the water, before the sun and the heat of the day evaporate the water. Watering your plants in the evening a few hours before sunset allows the soil to soak up the moisture, but it also allows time for your plants to dry off before nightfall. Many plant afflictions are spread while plants are wet for extended periods of time. An added benefit to watering your plants at night is that much of a plant growth occurs during the night. So having enough water in the soil during that time is important.
A dry or heat stressed garden will show subtle signs from lack of water even in the early morning hours, before the sun's heat is on the garden. Keep an eye out for water droplets at the ends of the leaves in the early morning. If you see these water droplets, that's a good sign that there is at least enough moisture in the soil to keep the plants healthy. These water droplets form when the plant adjusts the water pressure inside of itself. Any excess water that builds up overnight in the plant while the plant is sleeping and growing will come out at the ends of the leaves by a process called Guttation. These water droplets at the ends of the plant leaves should not be confused with early morning dew. The droplets from Guttation will have an overall pattern along the ends of the leaf as opposed to morning dew which will be covering the entire leaf. If you do not see these droplets in the early morning from Guttation, then your garden may be in a state of drought brought on by the high Summer heat.
A healthy Garden here in the Upstate should receive at least 1'' of water per week during summer. Of course their are exceptions such as Xeriscape gardens, which are usually designed to survive in areas of very high heat or where water is scarce.
Since making sure that your garden is well watered every week may not fit your busy schedule, that is where mulch saves the day. Mulch will help conserve the moisture in your garden during the high summer heat. To make sure mulch is helping to conserve moisture in the soil and not keeping water from entering the soil, water your garden well before you put the mulch down. Then mulch your garden to a depth of between 1''-3'' and water it again. After this initial two-step watering, your garden should be on its way to conserving moisture in the high summer temperatures. You may still need to water your garden, but not nearly as often.
Having a garden very full of plants not only is beautiful to look at, but it also keeps moisture in the soil. At first you would think that having a very full garden would take more watering, but it is just the opposite. Having a garden full of plants increases the humidity level in the garden. The garden plants also help conserve water in the garden by shading the ground from the sun's moisture evaporating powers. If you have spaces of exposed soil or mulch between plant species, it may look neat, but you are allowing the sun to shine directly onto the ground. This means that the water will leave your garden that much faster. And unless enough mulch is on the open ground between plants, the plants roots may be heat stressed as well.
Keep in mind that your entire garden is alive. So not only do the plants suffer in the heat, but the living food chain in your garden suffers as well. While parts of your garden's natural food chain may begin to collapse from the heat stress, other parts of that same food chain will take advantage of the collapse. This is where plants become more open to attacks from aphids and other disease spreading plant pests, while beneficial insects begin to decline as they leave to look for more favorable conditions.