So even though the weather forecaster isn't predicting a freeze, once temperatures start to drop below 40°F you need to be aware. 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.
40F won't hurt any of those plants but if the soil gets too cold they can all be affected. So if it's me I cover if I think the soil might get too cold too long. Most warm season crops I like to maintain 60F soil temp minimum.
Tomato plants can tolerate temperatures above freezing (over 32 degrees Fahrenheit, or 0 degrees Celsius). However, temperatures of 32 degrees Fahrenheit or below result in a frost or freeze, which kills unprotected tomato plants.
There are, of course, many variables that can affect winter hardiness in a plant, but in general, my rule of thumb is as long as the mercury stays in the 20's at night and rises above freezing during the day, our plants should be just fine.
Hardy plants can be hardened off when the outside temperature is consistently above 40° F. Half-Hardy plants may be hardened off at 45° F.
For best chance of survival, bring tender plants and annuals indoors before nighttime temperatures fall below 45 degrees. To be on the safe side, especially if you have tropical plants, bring them in when it's 60 degrees at night.
Hanging Baskets
If you have one of the tender plants in a hanging basket, it's best to bring it inside if the temperature is going to drop below 40 degrees F. You can also get away with storing them in a garage.
H2 Rating - 34 to 41° F (1 to 5 C)
Overall, these annuals are the most delicate of all the hardy annuals - one might call them half-hardy since they are really only hardy into the few temperatures just above freezing. And it's important to NOT let them freeze.
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.
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).
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.
Frost Hardy
Root crops develop more sugars when the soil temperature is below 40 degrees Fahrenheit; and Brussels sprouts are often best after a light freeze. Among the hardiest vegetables are cole crops, which can tolerate moderate freezes (24 to 28 degrees Fahrenheit) : Broccoli, cauliflower (26 degrees Fahrenheit)
Temperatures below 60°F at night will also result in blossom drop. At temperatures below 40°F, peppers may show some chilling injury. A shortage of water at bloom time can also result in blossom drop or failure to set fruit.
A local study done on frost formation relating temperature to dew point has these guidelines for frost: temperatures from 38 to 42 F can lead to patchy frost, 33 to 37 areas of frost, and 32 and below widespread frost/freeze.
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. Most garbage bags are made out of low-density polyethylene plastic.
Properly applied frost cloth can protect your frost sensitive plants from the cold even when the temps dip into the 20s! If necessary, the frost cloth can be left on the plants for an extended period of time without the risk of harming your plants.
When do I know it's safe to bring my plant outside? It's safe to move your plants outside when the outdoor temperatures stay consistently above 50°F. Pay attention to the weather report. If nighttime temperatures are set to fall below 50°F, bring your plants in for the night.
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.
Cover Plants
Once temperatures drop below about 28°F, covering is typically no longer effective at preventing cold damage. Occasionally even with covering you may still see damage at temperatures above 28°F, especially when the below freezing temperatures are prolonged (5 hours or more).
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.
For most flower types, optimal storage temperatures range between 33°F – 37°F while cold-sensitive blossoms and tropical flowers should be maintained at temperatures above 50°F.
Many newer cultivars are able to withstand hotter temperatures, but no petunias thrive in chilly temperatures. When spring temperatures dip below 40 degrees F, either cover your outdoor petunias with frost cloth or bring them indoors. Temperatures below freezing will kill them in a jiffy.
Plan to bring in your houseplants when temperatures are forecast below 45 degrees F to protect them from stress and cold damage. Tender perennial herbs like Rosemary or Lemon Verbena can be brought indoors as well for winter cooking.
The hardiest of flowers can be planted as soon as the soil in your garden can be worked, even if it's several weeks before the last frost of the season. For half-hardy flowers, hold off until a couple weeks before the final frost, and for tender flowers, plant when there's no chance of frost for the rest of the season.
Zucchini is a warm-season crop that cannot tolerate frost or freezing temperatures, so its best to plant your zucchini in the early summer, when temperatures are at least 70 degrees Fahrenheit or more.