The National Weather Service issues frost advisories when low temperatures are forecasted to be between 33-36°F. Frosts generally cause minor damage but can cause damage to sensitive species, especially flowers and flower buds that have broken dormancy. A freeze is more damaging than a frost.
Frost Advisory - This is when the temperature is expected to fall to 36 degrees to 32 degrees Fahrenheit. 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.
Covering plant material at temperatures between 39-30 with freeze cloth or covering can be effective. Prolonged exposure at temperatures of 28 degrees and below will notice protection results diminish with each degree dropped.
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. Some keel over when temps fall to 40°F; others crumble at 35°F. Other plants are just hardy by nature and can withstand temperatures as low as 18-20°F.
While it might be tempting to ship indoor plants outside at the first sign of spring, it's best to wait until nighttime temperatures stay at or above 60°F, and after any threat of frost has passed. If you're planning ahead, use the average last frost date for your area as your guide.
Most houseplants are native to tropical areas and will not tolerate freezing temperatures. Many indoor plants will see damage when temperatures drop to the mid to lower 40s°F and nearly all will die if exposed to below freezing temperatures.
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.
Damage is More LIkely When in Bud or Bloom
Overall, even flowers can tolerate light freezes (low 30s to upper 20s °F) and even a little snow. Record cold temperatures (below mid to upper 20s) will damage or destroy many of the flowers of early blooming varieties.
Another temperature to keep in mind: tomato plants will not survive in temperatures below 35 degrees Fahrenheit, and will not thrive at temperatures below 50 degrees. Gardeners can use a frost date calendar to note their region's average dates of last frost in spring and first frost in fall.
So, how hot IS too hot for plants? 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.
If using a good covering and the temperatures are expected to remain low, you can keep your plants covered for days.
Most peppers are not happy when temperatures drop below 50-60˚ F.
Cucumbers are heat-sensitive and do best in temperatures between 60-90°F (16-32°C). They can tolerate some heat but will become stressed if temperatures exceed 95°F (35°C). Cucumbers can also be damaged by frost and temperatures below 50°F (10°C).
As a general rule, tender plants should be brought in when nighttime temperatures are below 50 to 55 degrees F, even if they are hardy for your zone.
In general, when the wind chill is 32° and above, it's safe to be outside. In temperatures 13° to 31°, you should take indoor breaks every 20-30 minutes. For wind chills of 13° and below, move activities indoors as frostbite can quickly set in. How Cold Is Too Cold to Go Outside?
The National Weather Service issues frost advisories when low temperatures are forecasted to be between 33-36°F. Frosts generally cause minor damage but can cause damage to sensitive species, especially flowers and flower buds that have broken dormancy. A freeze is more damaging than a frost.
A moderate frost, with temperatures ranging from about 28°F to 32°F, necessitates a bit more precaution. In addition to tucking your plants in well with some frost cloth or cover, it's also a good idea to water your plants the evening before frost is expected to hit.
It's sunny again… wait… ), but some vegetables won't be happy (and grow into incredibly prolific tomato producers - yes, I'm talking to you, tomato fanatics) if you don't protect them until night temperatures warm up consistently to 50 degrees and above.
Each plant has a different threshold when it comes to dealing with the cold. Some plants can survive outside even when temperatures hit freezing. Others need to be brought indoors when temperatures reach the mid-40s. Be sure you know this before you plant.
For most species, actively growing tissue is damaged by brief exposure to temperatures above 45oC, while prolonged exposure can result in fatal injury. Temperatures between 30-40oC can be termed moderately high temperatures and result in reversible inhibition of metabolism (moderate heat stress).
If extreme heat continues for weeks at a time, plants can actually die from a depletion of their food reserves. High temperatures can also cause severe water loss (desiccation) when transpiration (the process by which leaves release water vapor to the atmosphere) exceeds moisture absorption by the roots.
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.
Move pots to cooler locations.
Place containers where they receive some shade during the hottest part of the day. This usually won't reduce their performance, but it will cool the plants and reduce water needs. Moving containers off or away from hot pavement will also help.
Temperatures at or below 60 degrees F can lead to nutrient deficiencies and slow growth. Consistent ventilation with fresh air whenever possible is the best way to keep temperatures within your plants' comfort zone.