Watering plants in the heat of the day is not ideal, but not because the water and sun together will scorch the leaves.
If the plants are in need of water and might decline due to drought, it's perfectly fine to water plants in the sun. We often hear about the dangers of watering in hot sun---whether the droplets will magnify the sun and cause burn spots (I've never seen this) or all the water will just evaporate (yes, some will)....
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.
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.
Hot or cold water will not benefit the plant in any way. Water that is hot (even the water in a hose that was left out in the sun all day) can scald the foliage and also harm the roots. While cold water can really damage the plant's roots, especially tropical houseplants.
The best time to water plants is in the morning or evening.
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.
The time needed to recover from stress is proportional to the severity of the damage. Plant heat stress recovery may take months for crops that have been ignored for too long during a heatwave; in extreme cases, it may be impossible to bring them back to life.
Plants in containers and hanging baskets may need water twice a day - especially when weather is windy and hot, containers dry out very quickly. Check often, as most will need water at least once a day, and some may need water in the morning and the evening.
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.
Water the soil, not the leaves.
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.
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.
Extra water now could pull them out of dormancy. These stressed plants may then expend a lot of energy growing a new set of leaves only to be faced with the likelihood of more heat and no natural rainfall. If they do not emerge from dormancy, they will be susceptible to fungal pathogens that grow in moist, hot soil.
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.
While most of our garden plants need at least 6 hours of full sun to be productive, too much sun, especially when combined with too much heat, can be too much for plants to bear. Problems arise especially when temperatures are over 85 to 90 degrees F.
Sunscald injury of plants is easy to prevent, though there is no cure. Once leaves are damaged, all you can do is support the plant until it manages to grow new, stronger leaves.
In absence of sunlight the green chlorophyll pigment gradually undergoes degradation and no new pigment molecule is formed, giving the leaves of the plant a pale yellow color.
Browning leaves are typically caused by under watering, sunburn, or overwatering. If the leaf tips are turning brown and crunchy, the soil likely became too dry for too long in between waterings. This can also cause the plant to drop leaves.
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.
The general answer is around 90 degrees F, with some exceptions to the rule. This means that when temps rise above 90 and remain there for a lengthy spell: Leaves wilt. Water evaporates into the atmosphere more quickly in high heat, draining a plant's reserves.
Outdoor plants are a great addition to any home or patio area. Figuring out the perfect balance of water to help keep them healthy and looking great is a must. Misting your outdoor plants is a great way to ensure you get enough moisture without overwatering them and without underwatering them.
Common signs of heat stress in plants include leaves with crispy, brown scorched edges, leaf drop, cessation of bloom, bolting, wilting, and rolled leaves. These are all common, natural ways that plants protect themselves from the heat.
The plant's appearance will perk up when new growth begins, and once new leaves have emerged, you could gently remove the scorched leaves if they have not already fallen off naturally.