But there is no doubt that watering in full sun is not water efficient – as much of it will evaporate before entering the soil. In the current hot breezy weather it is probably best to water in the early evening.
And most importantly, plants absorb water more readily when they're not under heat stress—a condition that's more likely to occur when the sun is at its peak. So while it's perfectly okay to water plants while they're in full mid-day sun, it's not the most efficient way to go about it.
Watering in the heat of the day shouldn't hurt the plants -- it actually cools them off -- but it's a far less efficient use of water as much of it will evaporate before reaching the roots. Avoid getting plants wet late in the day unless it's the only possible time you can water them.
Most plants shut down during extreme heat, so if it is over 100 degrees, it may be best to wait to water even if your plant is stressed. Throw a damp sheet over it, or mist it lightly to bring the temperature down.
The best time to water plants is in the morning or evening.
More importantly, watering at these times actually helps the plant retain water. If you water in the afternoon, especially during summer, the heat and sun are at their peak and the plant's water will evaporate instead of absorbing into the soil and roots.
During really hot weather, water your vegetables at least two to three times a week. Watering the garden deeply is critical. The water must go down, down, down to encourage deep roots and get away from the hot soil surface.
As the weather heats up and plants reach full size, you may need to water containers (and especially hanging baskets) twice a day. When you water containers (or plants in the ground), check that the soil is actually dry before you water.
In hot weather you need to water in the mornings when it is cooler. This will allow more water to reach the root system before it evaporates in the heat. There may be times when you cannot schedule watering in the morning. In this case, water the plants in the late evening before bed.
During extremely hot weather (daytime temperatures above 90F and nighttime temperatures above 70F), try to water daily or every other day. In a 10x10-foot garden, this would mean giving your plants 8 to 9 gallons of water each day.
Established Plantings
Use a soaker hose or sprinkler for 30-60 minutes at a time. During normal summer temperatures, this type of watering can happen every 2-3 weeks. In times of extreme heat, 1-2 times each week may be necessary. Know your plants though!
Avoid watering when temperatures are below 40°F or when sustained freezing temperatures are expected within 24 hours as this water will freeze and not be available to the plants.
To the gardening world it may have always been considered a fact, but science has never proved the widely held belief that watering your garden in the midday sun can lead to burnt plants.
If you forget or are too busy to water your plants on some summer mornings, you can water them at night. However, this should not be your ordinary watering time. Morning is the proper time for watering plants, though you should keep in mind that not all plants need daily watering.
If it's 70-80 degrees outside try watering 2-3 times a week. During the spring your lawn will be fairly easy to manage so you don't have to water all the time. In the blazing summer, when temperatures are between 90-100+, try watering 3-4 times a week.
Even in the warmest days of summer, a good soak every three or four days is best for plants because it forces their roots deeper into the soil where they are healthier and cooler.
The optimum temperature for roots to absorb water and nutrients is around 68°F. At that temperature, the water in the substrate still contains a lot of oxygen, and it is also exactly the right temperature to trigger the pump mechanism in the roots.
Mulch, mulch, and mulch some more!
Cover your soil with a blanket of organic material such as straw, leaves, shredded paper or cardboard, or bark. This will moderate soil temperature, prevent runoff and evaporation, and hold moisture in the for longer periods between waterings.
The best time to water outdoor plants is in the morning when temperatures are usually cooler. This gives the plants time to absorb the water so they can get through a long, hot day. The second-best time is late in the afternoon or early in the evening. Try not to water at night.
Higher temperatures cause the plant cells which control the openings (stoma) where water is released to the atmosphere to open, whereas colder temperatures cause the openings to close. Wind & air movement: Increased movement of the air around a plant will result in a higher transpiration rate.
If you forgot to water in the morning, there's still that window in the late afternoon, between 3-5pm. That time, when the sun is almost setting, is perfect because the temperature has already started dropping. In addition, there is also enough sun to aid in water evaporation.
In times of extreme heat, it is vital to water your lawn for about 30 to 45 minutes daily. Once the temperatures drop below ninety, you can cut back to watering three to four times a week, until that blissful thunderstorm comes and quenches your lawn's thirst!
If you leave your elbow in the water or 5–10 seconds, you'll be able to form a rough idea of the water's temperature. If the water feels slightly warm, but not hot, it's around 100 °F (38 °C).