An inexpensive soil thermometer helps keep planting time in perspective. “Fifty degrees is a good benchmark for cool-season crops,” Weston said. “And the soil should be 60 degrees or more for warm-weather plants like tomatoes, peppers and basil. In fact, for tomatoes it should ideally be 65 to 70.”
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.
Tomato seedlings suffer cold damage in temperatures below 60 degrees Fahrenheit. Some tomato varieties are bred to be more cold-tolerant and can set fruit when it's below 55 degrees Fahrenheit.
Cucurbit family temperature tolerances
Cucumbers can also be damaged by frost and temperatures below 50°F (10°C). Cantaloupes prefer warm temperatures and grow best in temperatures between 70-85°F (21-29°C). They can tolerate some heat but may become stressed if temperatures exceed 90°F (32°C).
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.
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.
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.
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).
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.
And any time the nighttime temps fall below 33°F, you can expect damage to the leaves and the plants may die. Should the freeze grow colder still, with temperature plummeting below 28°F, it's unlikely you can save the plant even using a row cover or plastic to protect them.
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.
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.
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.
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).
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.
Impatiens don't tolerate cold weather and will wilt and die once frost arrives. Don't plant impatiens outdoors until after the last frost of spring; it's best to wait until nighttime temperatures remain at 50ºF or higher.
Plants need to be transitioned indoors before the outside temperature starts dropping below 45 degrees Fahrenheit (7 degrees Celsius) at night.
Protecting Cucumbers from Cold
Even prolonged exposure to temperatures below 55 degrees F. (13 C.) can cause decay, pitting, and water-soaked areas on fruit. Sudden cold snaps can cause cucumber plant damage on leaves, stems, and fruit or even kill the plants.
Climate. Transplant outdoors after nighttime low temperatures are above 50°F. Dry soil, temperatures above 90°F, or night temperatures below 60°F or above 70°F, can weaken plant growth.
At temperatures below 40°F, tomato plants will begin to wilt and brown from chilling injury. At best, the plant will experience stunting, wilting, and pitting of fruit.
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.
Don't put tender seedlings outdoors on windy days or when temperatures are below 45° F. Even cold-hardy plants will be hurt if exposed to freezing temperatures before they are hardened. Reduce the frequency of watering to slow plant growth, but don't allow plants to wilt.
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.
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)
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.