The books are right – planting in the summer when it is hot and dry is not the best time to move new plants into or around the garden. During a particularly bad heat wave it can be hard enough to keep the soil hydrated, let alone provide ample moisture for plants that are establishing new roots.
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.
Many seeds will not germinate at all if the soil gets too hot. During periods of extreme heat, one option is to start these seeds indoors under lights, and then transplant them into the garden after hardening them off (gradually adjusting the plants to direct sunlight and wind).
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.
Leaves under heavy heat loading progress to senescence, brown-out and finally are shed. If unchecked, the heat stress syndrome results in progressively decreased photosynthesis, increased respiration, a major slow down in transpiration, and plant starvation through the rapid use of food reserves.
High temperatures mean a high potential for water to evaporate before it ever reaches the roots. Avoid sprinklers and instead use spot watering at the base of the plant, soaker hoses, or drip irrigation systems to provide water efficiently. Plants in containers and hanging baskets may need water twice a day.
It can take anywhere from a couple of days to a couple of months for stressed plants to recover. Recovery time depends on the amount of damage that the heat caused to the plant. If a stressed plant is completely neglected during a heatwave, it can take several months or even be impossible to revive it.
When a plant gets too much direct light, the leaves become pale, sometimes burn, turn brown and die.
It is so hot out there!
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. Mist leaves with a spray nozzle to cool down plant and reduce transpiration.
Most plants grow best in temperatures ranging from 59°–86°F. When temperatures above 90°F are sustained for long periods, plant growth is slowed, and some plants begin to show signs of stress.
Summer isn't as ideal of a time to plant as spring or early fall, but with a few precautions, most plants will do fine. Peak sunlight and searing heat are added stressors on young plants with small roots, not to mention, summer soil tends to be drier.
Depends on the plants and your goals. Most plants will be stressed out at 100+ but some tolerate it better than others. If I'm trying to grow things like lettuces in the heat, I will cover those even in 80s. Plants with strong established root systems will usually be OK whereas new plants can be more vulnerable.
Anything over 85 degrees is too much. For me, it's my comfort-level cut-off point, but for the plants it bodes a struggle-to-survive issue.
The general rule of thumb is that most plants freeze when temperatures remain at 28°F for five hours. Of course, there are exceptions to this rule. Seedlings, with their tender new leaves, often give up the ghost when temperatures dip to 32-33°F. Tropical plants have differing low-temperature thresholds.
The books are right – planting in the summer when it is hot and dry is not the best time to move new plants into or around the garden. During a particularly bad heat wave it can be hard enough to keep the soil hydrated, let alone provide ample moisture for plants that are establishing new roots.
As a native to the tropics, lantana likes it hot and humid, and grows best in moist, well-draining soil (but can withstand drought conditions). It thrives in the sun, especially afternoon sun, and blooms year-round in tight clusters of red, orange, yellow, pink, or white.
Most summers can be warm, but abnormally high temperatures can stress plants, resulting in sunburn, wilting, and even death. Plants in containers are more vulnerable to extreme heat and weather because the air temperatures can “fry” the roots and tops of plants.
Even sun-loving plants like cacti can develop dark patches from sunburn, for instance. Other signs that they're getting too much sun include drooping leaves and a faded color. The soil may become dry, the plant may be hot to the touch, and sometimes the leaves can even crumble to the touch.
In the presence of light, plants direct their growth toward the light. The growth that occurs at night tends to be vertical, unaffected by this daytime tendency to reach toward the light. Some plants may even grow faster at night because during daylight they focus their energy resources on photosynthesis.
One of the first signs will be wilting. As the heat begins to evaporate large amounts of moisture, your plants won't be able to actively replace all the lost water, and they'll end up with droopy, sad-looking foliage. Other plants will also “close” their leaves to reduce moisture loss.
Keep them in a shady spot to give them a chance to recover: even plants that look terminally crispy can often revive and re-sprout with this treatment. Larger pots should be heavily watered, then allowed to drain – one deep watering is better than daily sprinkles.
Remove affected leaves to allow the plant to redirect energy to healthy areas. Mulch around your plants to protect the root system, ensuring hydration and shielding from the sun. Avoid piling mulch around the tree's trunk. If your plant is in a container, relocate it to shade during the hottest afternoon hours.
In most cases, the shock is temporary, and with proper plant care, plants usually recover within a few weeks.