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)
Many tropical plants cannot survive temperatures below 40 degrees. If you cannot move your pots (with annual or perennial plants), or you have tender plants in the ground, cover your plants. The idea behind covering plants is to trap the heat from the soil and keep the cold air out.
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.
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).
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.
So will peppers survive 40 degrees? To sum it up, most peppers won't necessarily DIE from 40˚ F temperatures, but their growth can be stunted, and they may be slow to recover so it's best to keep them warm if possible.
Scientists in the ARS Photosynthesis Research Unit at Urbana, Illinois, have discovered a drop in the overnight temperature below 50°F can create a biochemical version of mission impossible for some crops like tomatoes, soybeans, and corn.
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.
Cucumbers are very tender, warm-season plants that grow best in temperatures from 65° to 75°F (18.3° to 23.9°C) with a minimum temperature of 60°F (15.6°C) and a maximum of 90°F (32.2°C).
For example, the optimum temperature range for tomatoes is 65° to 85°F (see previous chart). Within that range, it takes approximately 6 to 8 days before seeds germinate. Tomato seeds may still germinate at 50°F, but it will take over 40 days, and there will probably be no germination if the soil temperature is 104°F.
Tomato plants can suffer from chilling injuries when temperatures are between 0 °C to 5 °C . Pepper plants can also suffer from chilling injury if temperatures stay between 0 to 10 °C (32 to 50 °F ) for a long time. This injury may appear as puckered leaves and stunted growth.
When air is cooled to below the dew point, dew can be formed on surfaces; if temperatures are also below 36°F, the result is frost, which is frozen ice crystals. A light freeze is considered 28°-32°F, and a hard freeze below 28°F. However, any temperature below 25°F is dangerous territory for most vegetable plants.
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.
So when temperatures start dipping below 50º F, leave the tomatoes on the vine if warmer days and nights are forecast. But if temperatures have started on a steady downward slide, go ahead and pick the mature green tomatoes and bring them indoors.
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.
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.
However, warm-season vegetables such as peppers, tomatoes, cucumbers, melons and squash can be damaged if the temperature drops to 33 F.
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.
Outdoor cucumbers – harden off carefully, before being planting outside from early June, once you're sure temperatures won't drop below 12–15°C (53–59°F) at night. Choose a warm, sunny, sheltered spot.
First, it's good to know which plants are sensitive to temps around 35 or 40 degrees F. These are common tender plants that will need some protection. Hanging baskets: begonias, calibrachoa, gerbera, impatiens, vinca, euphorbia, and scoparia.
After that, the daytime temperature should be maintained at a minimum of 18°C, and the nighttime temperature at a minimum of 16°C (60°F) to ensure that enough male flowers are produced for adequate pollination. In the summer, daytime temperatures can get up to 23° to 25°C (73° to 77°F), which the crop can tolerate.
Basil is an extreme example of this, as the leaves can turn black when the temperatures dip into the 40s. Other plants are a little more tolerant, and warm temperatures following a short cold exposure can reverse some of the injury.
Peppers may also be grown very successfully in containers. Peppers thrive when temperatures are warm. Being sensitive to the cold, planting should be delayed until the danger of frost is past in the spring. Ideal temperatures are 70°F to 80°F during the day, and 60°F to 70° F at night.
Most winter squash have pretty good frost tolerance, as long as they don't get exposed to a hard freeze where the temperature might get down to 28° for more than a couple hours. If a heavy frost or freeze is predicted, you can cover your squash with old blankets or a tarp to provide some protection.
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.