Cover tender spring annuals: If you are expecting a frost in your area, cover your annuals overnight with light sheets or blankets, or burlap. I have some white sheets that my family doesn't use much, so I decided to use those to cover my annuals — marigolds, petunias and geraniums.
To know when to cover your plants, you need to know when the plants you have are susceptible to frost damage. Some frost-tender plants need to be covered as soon as the temperature hits 32°F, while others can handle lower temperatures and/or longer freezing periods.
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.
Semi-hardy cool-season crops (beets, carrots, Swiss chard, lettuce, cauliflower, potatoes, parsley) grow in minimum daytime temperatures of 40 degrees and higher. They cannot withstand hard frost without some kind of covering or protection.
So, keep an eye out for your local weather forecast and get your plant coverings ready when temperatures are forecast to dip to 32 degrees F and below.
Put coverings in place as soon as temperatures are below 36 °F (2 °C). Start the process of covering your plants for the winter when temperatures in your area fall to just above freezing in order to prepare them for the coming freezing temperatures.
As gardeners, our frost dates are based on 32 degrees Fahrenheit (0 degrees Celsius) to avoid the risk of any plant death. However, know that even if air temperatures are as high as 38°F, frosts may occur on the ground and on plants. It's better to protect plants just in case!
Some cool-season annuals can tolerate soil temperatures as low as 45 degrees Fahrenheit; however, most cool-season annuals will grow best in soil temperatures at 65 degrees Fahrenheit or warmer.
For many plants, temperatures below 50°F can cause problems. Adjust thermostats to cater to your comfort, but remember your plants need some consideration. Avoid placing plants near cold drafts or heat sources. Keep plants several inches away from exterior windows.
If you're wondering at what temperature threshold you should be bringing your plants, there's a short answer: when nighttime temperatures reach 45 degrees (F), it's time to bring your plants indoors.
Our recommendation would be to remove the cold protection covering once temperatures are above 32 degrees. If you leave the covering on when it gets warm and the sun is shining brightly, it may get too hot inside the cover and stress out the plants.
With that in mind, experts generally recommend flowers and other houseplants be brought inside or otherwise protected before the thermometer dips below 45 degrees Fahrenheit. For warmer-weather and tropical plants, that threshold is a bit higher, at around the 50-degree mark.
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.
To extend the beauty of your garden into autumn and put some color back into your landscape, consider planting some cold-tolerant annuals. Many of these can take temperatures to 20° F. As our seasons change, so can your garden by highlighting these beautiful tough plants in your beds, borders and containers.
Tender plants, such as tropical houseplants and geraniums, are killed when the air temperature stays below 32˚F for a few hours. A freeze warning often signals the end of the growing season in fall because temperatures are low enough to kill off annuals and begin dormancy for hardy perennials, trees, and shrubs.
Avoid watering when temperatures are below 40°F or when sustained freezing temperatures are expected within 24 hours as this water will freeze and not be available to the plants.
During winter, geraniums grow best with night temperatures of 50° to 60°F (10° to 16°C) but will survive if they drop to 32°F (0°C) and/or rise above 80°F (27°C), as long as they are kept relatively dry. When new growth appears in the spring, cut off all the old leaves.
Signs Your Plants May Be Too Cold
The leaves brown and fall off. If your plant is too cold, it may begin to die, and its leaves will brown and fall off. The leaves turn yellow.
Most potted or hanging plants, like petunias, are fine as long temperatures don't dip below 39 degrees, and especially if the temperature doesn't dip to the freezing mark.
Temperatures below 32 degrees can freeze the leaf tissue of cold-wimpy plants and turn them to mush after just a few hours. Some of the most tender annuals and veggies might not die but suffer cold-induced setbacks even when overnight lows dip below 40.
Petunias thrive when nighttime temperatures are in the 55 to 65°F range, and daytime temps are between 61 and 80°F. They will succumb to sub-freezing temperatures in a hurry, and anything below 40°F may kill them, though Wave® petunias can tolerate temperatures as low as 35°F.
If a sudden cold snap shows up in the forecast after you've planted, you can always cover them overnight to be on the safe side. If you do cover plants - be it new or tender perennials or annual flowers or vegetables - cover only overnight. Remove your covering once the temperature goes above freezing the next day.
A covered porch usually provides protection from light frost, but the garage or sun room is better for freezing temperatures. A couple days in darkness won't hurt the plant. Or move them out during the day and back in at night, if cold temperatures persist.
From that experience, I've found the best frost protection for your outdoor plants is either free or cheap. Cardboard boxes and brown grocery sacks make perfect frost cover and at the end of the season can be recycled. I keep various boxes on the patio and when frost is forecast simply put one over the plant.