“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.”
"The ground temperature is still pretty warm so when you do cover your plants the heat from the ground actually helps protect them as well," Ewing said. When it comes to what flowers or plants could be in jeopardy, it varies. "Your topicals, they're not going to like it below 50.
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.
Many plants grown in the garden will be damaged at temperatures below freezing. Watch the weather forecast for temperatures at or below 32 degrees Fahrenheit.
Most houseplants are tropical plants that can safely grow in a heated home during the winter. They will never survive outdoors in winter unless you live where the temperature stays at least above 50 degrees. In northern zones 4 and 5, plants need to survive months of below freezing weather every winter.
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.
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.
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.
Cover Plants – Protect plants from all but the hardest freeze (28°F for five hours) by covering them with sheets, towels, blankets, cardboard or a tarp. You can also invert baskets, coolers or any container with a solid bottom over plants. Cover plants before dark to trap warmer air.
Summer squash, such as zucchini and yellow squash, thrive in warm temperatures between 70-95°F (21-35°C). They can tolerate some heat but may become stressed if temperatures exceed 100°F (38°C). Summer squash can also be damaged by frost and temperatures below 50°F (10°C).
So, yes, it's been cold at night around Southern California, but folks, 45 degrees is a still a long way from the freezing temps that can burn our tender tropicals and other plants, like a multiday cold snap in 2007 that severely damaged crops in Central and Southern California.
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.
If temperatures remain warm both day and night (at least 50 degrees Fahrenheit), the seedlings should be able to handle increasing amounts of sunlight and can be left outdoors overnight. Make sure the soil doesn't dry out if temperatures should suddenly turn warmer.
No Frost Tolerance
Sweet Corn. Potatoes. Pepper, eggplant. Cucurbits: Cucumbers, summer squash, melons (pumpkins and winter squash may be able to tolerate very light frosts, but it is better to protect them if possible).
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.
Depending on the species, temperatures dipping below 45-50°F can cause damage. Here are some things to consider before you bring your houseplants back indoors.
Normally, it is best to keep tropical houseplants above 50 degrees F, but 60 F is even better. My basement is generally in that range, but with the severe cold spells we have had this winter, the temperatures have dropped into the upper 40s. The other important thing to remember is that heat rises.
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.
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.
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.
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.
Below 63°F, cucumbers do not grow. In such conditions, plants sit in the soil and become susceptible to damages caused by pests in the soil. With that said, check soil temperatures and be sure they are above 63°F before planting cucumbers.
According to Myers, the hardiest vegetables that can withstand heavy frost of air temperatures below 28 include spinach, onions, garlic, leeks, rhubarb, rutabaga, broccoli, kohlrabi, kale, cabbage, collards, Brussels sprouts, corn salad, arugula, fava beans, radish, mustard greens, Austrian winter pea and turnip.
The ideal temperature for tomato cultivation is between 21-27°C (70-82°F). This range is also best during pollination. These temperatures are considered optimal for day time. During the night, tomato crops may enjoy lower temperatures, reaching 16-18°C (62-64°F).
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.