In cold weather, water should be trickled slowly into the soil. Water only when air temperatures are above 40 degrees F.
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.
Freeze Warning - This is issued when there is at least an 80% chance that the temperature will hit 32 degrees Fahrenheit or lower. Light freeze - 29° to 32° Fahrenheit will kill tender plants.
The evening
Don't do that. Evenings are best avoided because of evaporation and transpiration (I think of transpiration like a plant breathing). Around 99% of water a plant takes up does NOT stay in the plant. Tiny pore-like holes on leaves, called stomata, take in carbon dioxide and 'breathe out' oxygen and water.
If it's very hot, watering during the hottest part of the day may not be the most efficient time to water, but it will not harm them. The idea that watering during the mid day will harm plants is a myth.
Morning watering is actually preferable to evening watering as the plant has time to dry before the sun goes down. At night, water tends to rest in the soil, around the roots, and on the foliage, which encourages rot, fungal growth, and insects.
Trees, shrubs, perennials and most of the “permanent landscape” are fine when gardeners go away for a week or two of summer vacation. Potted plants and hanging baskets are more of a concern. These can wilt after just a day or two without water, especially in a hot, sunny spot and especially when recently planted.
It is a long-held idea that midday watering will scorch plants' foliage and damage their health – but this isn't supported by the evidence, says James Wong.
The best way to tell if your plants need water is to stick your finger about an inch into the potting mix—if it feels dry, break out the watering can. If you detect dampness, check back again in a day or two. For smaller houseplants, you can also pick up the whole container. If it feels light for its size, add water.
Run your sink into a watering can, cup, or bucket, and let it sit for a good 24 hours. This will allow chemicals like chlorine and fluoride the time to evaporate from the water. We like to have a full watering can ready to go with still water so that if our soil seems dry, we can water our plant without waiting a day.
In general, a frost (31-33 degrees) will kill beans, cantaloupe, cucumbers, eggplant, okra, pepper, potatoes, sweet potatoes, squash, tomatoes and watermelon. These are warm-weather plants anyway and most are recommended for planting through September.
If a low temperature of 29o F is predicted, start with a row cover or frost cover. If temperatures are expected to drop several degrees lower, add a frost blanket or a layer of plastic sheeting over the top that will hold in the air and reflect heat back to the plant and soil.
Thoroughly Water plants if it won't rain before the freezing temperatures arrive. It may sound illogical. However, the moist ground stays warmer than dry soil. Watering the night before the freeze will insulate the grass and plants' root structure and decrease the potential for cold injury.
The root of the trick's protection is a concept called "latent heat of fusion." When the water on the plant starts to freeze, it releases heat (a necessary process to turn water into ice), and that heat helps protect the plant.
Frost penetrates deeper into the air spaces of dry soil than moist soil because, in moist soil, water has filled the air pockets. Hannah suggests that even if temperatures are at freezing and the pot is dry, it's important to water because it will help to better protect the plants' roots.
Ice cold water will cause root shock, which may lead to permanent root damage, leaf drop and other problems. Allow the water to warm to room temperature before watering the plants.
Improper irrigation is probably the most common cause of plant damage. Inadequate water amount causes foliage to wilt, discolor, and drop prematurely.
Leaves Turning Bright Yellow or Mushy Stems
This is usually a sign of overwatering, either caused by watering your plant before it's ready or if your plant is in too low of light. When plants sit in waterlogged soil, their roots can't get enough oxygen, leading to root damage which manifests as yellow leaves.
Provide Slow, Deep Watering
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.
Don't Water at Mid-Day in the Sun. "Don't Water Your Plants in the Middle of the Day!" How many times have you heard that? "Only water your plants in the early mornings or late evenings." THE CHALLENGE: That's easy to say, but not always easy to do.
Water the soil, not the leaves.
Browning of leaf margins and/or yellowing or darkening of the areas between the main leaf veins are symptoms of leaf scorch. Due to environmental causes, leaves may dry, turn brown, and become brittle. Look for damage to trees and shrubs on the upper portion on the sunny, southern side, and on the windy side.
Mornings and evenings are excellent times to water gardens when using a drip irrigation system or soaker hose. Watering in the evening isn't a problem as these methods don't wet plant foliage.
Deep watering entails watering at a slow pace for long enough that the water soaks deep into the soil, where the roots are. Often this means watering for 30-60 minutes at a time, 2-3 times each week, until plants are established.