Light freeze - 29° to 32° Fahrenheit will kill tender plants. Moderate freeze - 25° to 28° Fahrenheit is widely destructive to most vegetation. Severe or hard freeze - 25° Fahrenheit and colder causes heavy damage to most 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.
When temperatures dip below 50-55, I bring my plants in. This is my preference, but most houseplants will tolerate temps down to 40-ish. I'd say anywhere between 45-55 degrees is a good time to bring them in. This is when most pests and bugs have either died off or gone south for the winter.
The National Weather Service (NWS) says that a frost can form when the temperature falls below 36 degrees Fahrenheit, with it being more than likely in rural areas. Frost becomes more widespread when the temperature falls below 32 degrees. This is where freeze can occur.
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.
Water freezes at 32 degrees or colder, but multiple factors impact whether pipes will freeze. Pipes in 20 degrees or colder for prolonged periods will freeze. Exterior pipes freeze much faster than internal pipes. Insulating hot water pipes can raise the water temperature by 2 to 4 degrees.
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.
Avoid exposing plants to the freeze/thaw cycle.
To avoid this, place pots on soil instead of pavement. For plants in the ground, the main source of heat for roots is the heat of the earth. For containers on pavement, the sun can warm the pavement considerably, elevating the temperature of the rootball.
There is no universal temperature for all plants that is too cold, so that is why it is important to do your research. However, it is safe to say if living in an environment with harsh winters and temperatures below 20°F, it is always wise to bring plants indoors during this time, if possible.
A light frost may cause minimal damage while a severe frost may kill plants. Young, vulnerable plants are much more susceptible to a light freeze, which occurs when temperatures are 29 to 32 degrees Fahrenheit, while mature plants may only suffer from short-term effects.
A light freeze (between 32 and 29 degrees F) can kill tender plants. A moderate freeze/hard freeze (between 28 and 25 degrees F), can cause wide destruction to most plants. A severe freeze/killing freeze (24 degrees F and below), causes heavy damage to most plants.
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.
If using a good covering and the temperatures are expected to remain low, you can keep your plants covered for days.
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.
A "killing frost" is now considered to be a temperature of 28°F in the United States, but in 1916, the type of frost was determined by the level of damage incurred.
Despite its convenience, there's one item you should stay away from to cover your plants. If you're looking for tips for protecting your plants from frost, avoid using garbage bags as plant covers. Trash bags are not the best protection against frost damage, primarily due to their material.
On a cellular level, this means that as the water turns to ice, it expands, rupturing the cell wall. This is permanent damage, but it doesn't mean the plant will die. If the damage is isolated to a portion of the plant, it can be revived and recover.
Once temperatures drop below 40°F, you may want to start covering your plants with a frost blanket depending on what you're growing, which we'll get into in just a bit.
And this can damage or kill your plants. If the moisture conditions are right for frost, you can get it when temperatures reach 36 degrees or below. As I mentioned earlier, between 36 and 29 degrees, frost often has the biggest effect on your tender annuals.
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.
At a water temperature of 32.5 degrees, death may occur in under 15 - 45 minutes. At a water temperature of 32.5 to 40 degrees, death may occur in 30 - 90 minutes. At a water temperature of 40 to 50 degrees, death may occur in 1 - 3 hours. At a water temperature of 50 - 60 degrees, death may occur in 1 - 6 hours.
A frost can occur anytime temperatures drop to 37° or lower. Most often, frosts are associated with clear skies and calm winds overnight and during the early morning hours on a cold day. On the other hand, a freeze only occurs when surface temperatures drop below 32°, or the point at which liquid water freezes.
Your pipes are at risk of freezing any time temperatures drop below 32°F. However, it's much more common for pipes to freeze when temperatures dip below 20°F. The longer the weather remains below freezing, the more likely it is your pipes can freeze if not properly protected and winterized.
Because that water freezes at 32°F (0°C), a couple of degrees change in temperature can mean an entirely different form of precipitation. For example, a temperature is 34°F (1°C) means that precipitation will fall as rain. A few degrees colder could mean a snow day! Look at the graphs below.