The general answer is around 90 degrees F, with some exceptions to the rule.
You can fertilize whenever. It's a myth that you can't fertilize in high heat. You just need to ensure you're watering sufficiently - 1.5 inches per week ideally in this heat.
Vital considerations for determining when it's too hot to apply fertilizer include avoiding fertilization when daytime temperatures consistently exceed 85-90°F (29-32°C), as high temperatures can stress the grass and increase the risk of fertilizer burn.
I personally don't fertilize anything planted in the ground or houseplants from October 1 to March 1. If I plant veggies in January, I add plenty of compost to the soil to feed them through until March. Since plants don't grow that much in cooler weather, they don't use that much fertilizer.
What Temperature Is Too Hot for Plants? For the majority of plants, any temperatures above 85 ºF (29 ºC) start becoming uncomfortable. 90 ºF (32 ºC) is considered the plants heat stress baseline temperature.
It varies with species of plant and what they've been acclimatized to. But it's safe to say many plants shut down at 105 degrees. At various tipping points, plants close their stomata and stop photosynthesis. They're trying to hold on to water, so they don't use their energy for anything else.
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.
Always follow the manufacturers recommended rates and application methods. Fertilize outdoor plants in the coolest part of the day, such as early morning or late evening, especially in hot summer weather. Always water in fertilizers well to distribute nutrients throughout the soil profile and to prevent burning.
Perennials that require no fertilizer: Included are ornamental grasses, false indigo, ground covers, butterfly weed, bee balm, coneflowers, black-eyed Susans, sea holly, dianthus, asters and veronica.
Timing: Apply summer lawn fertilizer once between June and August, six to eight weeks after the late spring feeding.
Either too low or too high thermal degrees kill soil-dwelling organisms and plants. In particular, crops develop slowly at 90°F (32°C), while 140°F (60°C) is critical because bacteria in the ground can't survive the heat.
If you're not supposed to fertilize during heavy rains, you're also not supposed to fertilize on scorching days. During these days, lawns tend to lose their ability to retain moisture. When this happens, your lawn becomes drier and less able to absorb nutrients.
It is wise to be wary of fertilizing your lawn during hot weather. The best time is when temperatures remain below 85 degrees Fahrenheit with overcast conditions, using a slow-release formula. Extra fertilizer in the heat can burn the lawn and the extra flush of growth may have difficulties in the summer heat.
You should take a break from feeding your plants during a heat wave or drought. However, to prepare them for conditions that could cause heat stress, regularly give them Miracle-Gro® Water Soluble All Purpose Plant Food (following all label directions) so they're nice and strong before the unfortunate weather arrives.
1. Nutrient Absorption and Grass Health. Fertilizing your lawn when the temperature is too low (below 50°F for cool-season grasses and below 70°F for warm-season grasses) can lead to poor nutrient absorption.
Some garden plants are heavy feeders (meaning they need more nutrients than others). These tend to be species that grow fast and bloom a lot, including most annuals, fruits, veggies, roses, and hydrangeas.
As a general rule of thumb, you will usually fertilize perennial plants at least twice a year: in the early spring when new growth begins to emerge, and again in early fall. Too, any time you cut a perennial back during the active growing season, a feeding is usually beneficial.
The fact is, though, that you may or may not need to add fertilizer to get your plants to grow healthy. It is actually more likely than not that the level of nutrients in soil is perfectly adequate for healthy plant growth.
Fertilizing In Hot Weather Can Actually Harm Plants. Fertilizing your plants in hot weather might seem like a good idea, but it can actually cause more harm than good. Learn how to keep your garden thriving without the risk of fertilizer burn or nutrient overload.
Best case scenario, this is the standard schedule your lawn needs: Mid to end of APRIL – spring fertilizer – 32-0-6 slow release (fast acting) Early to mid JUNE (6 weeks later) – early summer fertilizer – 25-0-5 – 50% slow release. Mid to late JULY (6 weeks later) – late summer fertilizer – 25-0-5 – 50% slow release.
Granular fertilizer needs the help of water for it to activate, so it will have no effect on your garden if you forget to water after adding it. For liquid fertilizer, you should wait longer before watering your plants. In fact, you can wait for as much as 12-24 hours to water your plants again for the best results.
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.
Though some species are more cold tolerant than others, most will perish or enter diapause once temperatures fall below 32°F, which answers the question, “how cold is too cold for mosquitoes?”
Regularly misting the plant or placing it on a pebble tray with water can help meet its humidity requirements. Temperature: It prefers warmer temperatures, ideally between 65-75°F (18-24°C).