What temperature is too cold for plants? When it comes to deciding how cold is too cold, it really comes down to the plant itself. Many evergreen trees withstand temperatures as low as -30 or -40 F, while more tropical plants will only tolerate temperatures as low as 30 or 40 F.
Most plants prefer a temperature of no less than 60° F, though many can tolerate as low as 40° F. Along with this, while the improper temperature can certainly be an issue, fluctuations in temperature are the true killer. Learn the ideal temperatures for your green friends so they can be healthier and happier.
Regardless of which hardiness zone you live in, certain plants do tend to do better in cold weather than others. Flowers such as violas and crocuses are good options. So are snowdrops, hellebores, and heather. Even some types of roses can grow and bloom in winter.
Pansies and violas bloom for most of the winter in mild climates, and some types will rebound in the spring in cold climates. They're technically annuals, but many varieties drop seeds so they'll come back again next spring.
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.
Light freeze - 29° to 32° Fahrenheit will kill tender plants. Moderate freeze - 25° to 28° Fahrenheit is widely destructive to most vegetation. Severe or hard freeze - 25° Fahrenheit and colder causes heavy damage to most plants.
So even though the weather forecaster isn't predicting a freeze, once temperatures start to drop below 40°F you need to be aware. Once temperatures drop below 40°F, you may want to start covering your plants with a frost blanket depending on what you're growing, which we'll get into in just a bit.
Perennial Flowers: Most perennial plants do not suffer frost damage and do not need to be covered. Frost sensitive perennials include Hostas and Bleeding Hearts. They should be covered to protect their foliage and flowers. Cover any blooming or budded up perennials.
An important factor in the chance of survival is how long the plants were exposed to cold temperatures. A few hours can do the job, depending on the plant. Generally, though, it takes 12 to 24 hours of exposure to cold temperatures to completely kill most tropical plant species. If you're in doubt, check the roots.
It should be noted that some house plants are more vulnerable to cold damage than others and when the thermometer dips – they can suffer. Some of those more sensitive to cold temperatures: Orchids (Phaleanopsis sp.), Mandevilla, Bougainvillea, Oriental Hibiscus, and Poinsettia.
Perennials. The newly emerged growth of most perennials can tolerate temperatures in the low 30s and upper 20s °F. This includes many early emerging perennials such as bleeding heart, daylily, catmint, and columbine. In most cases, early leaf growth on these plants will be just fine with cold temperatures.
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.
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.
Thoroughly Water plants if it's not going to rain before the freezing temperatures arrive. It may sound illogical. However, a moist ground stays warmer than dry soil. Watering the night before the freeze comes will insulate the root structure of the grass and plants and decreases the potential for cold injury.
The cold can mix badly with water, so just make sure the night time watering happens when temperatures are warmer. Late Spring, Summer and early Fall / Autumn generally have warmer months and this tends to mean overnight temperatures are warmer too.
Depending on their hardiness, some potted plants will respond to the first frost by going dormant just like garden plants do. However, as the temperature continues to drop, their roots might die unless they are protected.
Take Special Precautions for Potted Plants
To protect them, you can wrap the pots in an insulating material (think burlap, old blankets, or even bubble wrap), place them close to the foundation of your house, and arrange them close together. You can also put a layer of mulch over them for added protection.
Snowdrops. Galanthus nivalis, commonly referred to as snowdrops, grow well in zones 3 through 7. They are one of the earliest blooming flowers in the spring, often peeking up through the snow as early as February. They do best in well-drained soils and are well adapted to rock gardens.