Understanding Temperature for Plants

If you want your plants to thrive, you need to pay attention to the temperature they live in. Learn how to determine what temperature your plants need both indoors and outside so you get the best results possible.

Plants are picky about their living conditions, and the wrong temperatures can make it hard for them to thrive. Understanding the role temperature plays in plant growth makes it easier to ensure your plants get the best start in life possible. Here’s what you need to know.

Plants, Temperature, and Photosynthesis

Temperature is considered second only to light as the most vital component for plant growth, and it’s a critical factor in photosynthesis. During this process, plants pull energy from the sun, which they transform into glucose to use as fuel. Temperature extremes on either end of the spectrum interfere with this process.

If the temperature drops too low, your plant may stagnate and die, especially if exposed to lots of cold water. Likewise, high temperatures will speed up plant respiration and can break down existing sugars, leaving the plant with little food to support further growth. This causes it to drop its leaves and become spindly.

Understanding Temperature Ranges

Temperature’s overall effect on plants varies based on many factors, including its sunlight exposure, elevation, temperature variation between day and night, the proximity to surrounding heat-retaining structures (like rocks and walls), and the moisture drainage levels in the soil.

Thankfully, plants are relatively resilient to most temperature fluctuations, especially between daytime and nighttime. In fact, flowering plants often do best with daytime temperatures between 70-80 degrees F (21- 26°C) but nighttime temperatures closer to 55-60 degrees F (12-15°C). These lower nighttime temperatures help the plant recover from moisture loss, which leads to longer-lasting flowers with more intense coloring.

Outdoor plants tend to handle temperature changes better than indoor ones. Here’s a general rule for the minimum temperatures most annual plants can handle outside.

However, each plant species has its own temperature requirements. When buying new plants, it’s important to research what maximum and minimum heat level they can best handle.

Indoor Temperatures for Plant Health

Growing plants indoors gives you more control over their environment, but this coddled environment means they can become more sensitive to temperature fluctuations over time. Thankfully they have a larger comfort range than most people do. People usually feel best in indoor temperatures between 72–82°F (22–27°C), but plants have a broader comfort temperature range of 58–86°F (8–30°C).

When choosing indoor locations for indoor plants, pay attention to potential temperature danger zones. These are places where the temperature can jump from extremes quickly, such as windowsills and drafty doorways. Long-term exposure to these spaces can stress your plants, so choose locations where the temperature won’t vary as much.

If your indoor plants show signs of stress, there are two primary options for promoting growth again. These include either increasing the available lighting (to promote photosynthesis) or decreasing the nighttime temperatures (to promote respiration).

Indoor Germination

While indoor plants will do best if you keep their temperature stable, the rules change slightly if you’re starting seeds for plants that will eventually move outdoors, like for a vegetable garden. In this case, you want to subtly mimic the variation of day and night temperatures so the plants don’t go into shock when planted outdoors.

It’s also possible to mimic the seasons indoors for plant germination. For example, many seeds need to be cold stratified before planting, meaning you need to keep the seeds in the refrigerator for several weeks to replicate winter temperatures before planting them in the “spring.”

Germination rates increase as the temperature goes up, but only to a point. Get it too hot indoors, and seedlings might suffer after sprouting. Some cool-season vegetables, such as broccoli and lettuce, require chillier temperatures between 55–70°F(13–21°C), while warmer weather plants like squash and tomatoes prefer to germinate in temperatures between 70–85°F (21–30°C).

The Importance of Outdoor Soil Temperature

If you’re growing plants outdoors, the soil temperature is a crucial factor in success. All soil microorganisms have optimal temperatures in which they operate, and ground that is too chilly or hot keeps them sluggish, which may negatively affect your plants.

You can measure the ground temperature with a soil thermometer to ensure you plant at the right time. For best results, use it in the early morning when the soil is coolest, and get the reading between one and three inches down in the ground. Take the temperature for three consecutive days and average the results for the most accurate reading.

If you need to warm the soil up quickly, covering it with dark mulch will attract sunlight and bring up the temperature within a few days. Plastic sheets work even better because the synthetic material holds heat in. You can also till the first few soil layers to bring it to the surface and promote excess water drainage that could be cooling it down.

Sometimes, waiting for the soil to warm isn’t realistic, and you need to get seeds in the ground fast. Here’s a closer look at the minimum and optimal soil temperature requirements for different plant varieties so you can assess when planting makes the most sense.

Minimum Soil Temperatures for Germination:

35°F (1°C): onion, lettuce, spinach.
40°F (4°C): broccoli, carrots, Brussels sprouts, cabbage, collards, Asian greens, kale, radish, Swiss chard, peas
50°F (10°C): celery, asparagus, corn, tomatoes.
60°F (15°C): cucumbers, beans, eggplant, peppers, squash, watermelon.
75°F (23°C): eggplant, okra, pumpkins.

Optimal Soil Temperature for Near-100% Germination:

65°F (18°C): parsnips
70°F (21°C): spinach
75°F (23°C): lettuce, asparagus, onions
80°F (26°C): carrots, beans
85°F (29°C): cabbage, beets, radishes, eggplant, peppers, Swiss chard, tomato
95°F (35°C): watermelon, corn, cucumber, squash

Is Your Temperature Wrong? Three Signs of Problems

Sometimes you might not notice problems with the temperature around your plants until you see signs of stress around them. Here are three ways plants express something is wrong with their growing conditions:

Note that each of these plant conditions could be caused by other problems unrelated to temperature. Observe the problem carefully and experiment with different changes until you see what makes an improvement.