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).
Use Garden Covers to Guard Against Frost. When the temperatures start dipping into the 30s overnight and frost looms, it's a great time to think about covering your vegetable garden.
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.
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.
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)
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.
Frost-Tolerant Vegetables
Frost-tolerant veggies can survive several hours in the 28 to 32 degree F. (-2 to 0 C.) range, providing they have been preconditioned to cold weather.
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.
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).
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.
It is best to do this when the earth is moist so that the soil under the plastic is damp and will hold the moisture.
If you placed some of your indoor plants outside for the summer, you will need to move them back indoors before temperatures get too cold. When nighttime temperatures start to dip consistently to around 50°F, it's time to bring plants back indoors.
Veggies & Fruits
Vegetables typically require daily watering during summer weather, but may need a second watering when temperatures are extremely high. Lettuces and other leafy crops can be kept from bolting (going to seed) by misting or watering their leaves in the afternoon.
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.”
Shade covers for vegetables are easy to make using cloth or fabric row cover, or you can use knitted mesh fabrics made for use as shade cloth. Their advantage over many other materials is hail resistance, which is no small matter in late summer.
Plants do best when watered deeply about three times a week, factoring in the rain. If the plants are seedlings, water them twice a day until established.
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.
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).
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.
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.
In the case of peppers, the ideal daytime temperature is 21-26°C (70-80°F). The ideal nighttime temperature is 18-21°C (65-70°F). Of course, these ranges may vary, depending on the pepper varieties you grow. Pepper plants may still grow outside of the ideal temperature range.
How Cold Can Tomatoes Tolerate? Tomato seeds won't germinate when the soil temperature is below 50 degrees Fahrenheit. 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.
Very tender vegetables should not be planted until at least three weeks after the last frost in spring. These crops demand warm temperatures to grow, usually above 70°F (21°C). They can be stunted by temperatures below 50°F (10°C).
Basil Cold Tolerance
So what temperature is too cold for basil? Basil really begins to suffer when temperatures drop into the 40s, (4 C) and by the first frost, is usually on its last legs.
Hardy vegetables can tolerate harder frosts and temperatures between -5°C and -10°C. Semi-hardy vegetables will tolerate light frosts and temperatures around freezing (0°C).