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!
Both annual and perennial plants in the ground benefit from a good deep soaking up to three times a week. Allowing the water to penetrate slowly and fully into the soil (to a depth of at least 6 inches) encourages roots to grow deeper and stronger and protects them from the parched soil surface.
When. Hands down, the best time of day to water is early in the morning. Watering during the day can increase risk of disease. If you can't water early in the morning, water late in the evening, especially before days forecasted to have dangerously high temps.
Usually when the first inch (2.5 cm.) or so of soil is dry, it's a good indication that watering is needed. In summer, watering outdoor potted plants is necessary daily (and even twice a day) for most species, especially when temperatures reach over 85 degrees F. (29 C.).
When the air and soil temperatures consistently fall below 40 F, it's time to stop watering. The ground can't absorb water once the top few inches freeze. Continue to water your plants up until this point so they're as well-hydrated as possible going into winter.
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.
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.
Avoid full sun
In the current hot breezy weather it is probably best to water in the early evening. This gives the plants enough time to dry out, but there is still the chance for overnight water uptake by the roots. And if you want to water in the morning then start very early – before the sun is shining.
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.
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.
If the soil feels dry, it's time to water. In the spring and fall and in cooler climates, it might be sufficient to water container plants every two to three days. In the summer and in warm climates, container plants usually require daily watering, unless they are succulents or other drought-tolerant xeriscape 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.
Dehydration is a primary contributor to heat exhaustion. Your work performance may suffer when you are dehydrated, even if you don't notice. When working in the heat, drink 1 cup (8 ounces) of water every 15–20 minutes. This translates to ¾–1 quart (24–32 ounces) per hour.
Water gardens early in the day when it's cooler and less windy. 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.
Overwatering is one of the more common causes of plant problem. Heavy and poorly drained soils are susceptible to becoming waterlogged. Roots growing in waterlogged soil may die because they cannot absorb the oxygen needed to function normally. The longer the air is cut off, the greater the root damage.
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.
As the weather heats up and plants reach full size, you may need to water containers (and especially hanging baskets) twice a day.
An ideal humidity level for most adult plants is 60% to 70%. Some tropical plants are used to humidity levels of up to 90%. Many succulents, such as cacti, only need 10% humidity. As a general rule, plants with thicker leaves can tolerate lower humidity.
Light freeze - 29° to 32° Fahrenheit will kill tender plants. Moderate freeze - 25° to 28° Fahrenheit is widely destructive to most vegetation. Severe or hard freeze - 25° Fahrenheit and colder causes heavy damage to most plants.
Use shade cloth or protective row covers.
Sensitive plants like salad greens may require a 50 – 60% shade factor, while more heat tolerant plants like squash and beans may benefit from a 30% shade cloth. A shade cloth is temporary but care should be taken to position it to block sunlight while not reducing aeration.
Watering every day is not advised. It should be done 2 to 3 times per week only to prevent a shallow root system. If possible, water during the early morning hours to ensure that the lawn dries entirely before nightfall. However, watering at any time of the day is better than not watering at all.
If you are unsure how long to water new plants, aim for 30-60 seconds for small plants – longer for larger plants while moving the hose to a few locations around the plant. Avoid watering when the soil feels moist. The earth must be allowed to dry out between watering.
Most outdoor plants can go without water for about a week. Container plants are the thirstiest. Most need to be watered daily. If you're planning to be away, adding mulch around flower and vegetable beds helps keep moisture in longer — even if it doesn't rain during your absence.