Photosynthesis only happens in the right temperature conditions. So, it is essential to find the right temperature.
The temperature is a critical factor in the growth and development of a plant. With light intensity, carbon dioxide content, air humidity, water, and nutrients, the temperature influences plant growth and is one of the decisive factors for deciding if it’s safe to plant outside. All of these factors should be in balance with one another.
What Temperature is it Safe to Plant Outside?
The temperature required to plant outside varies from plant to plant. Each of them has a minimum temperature and a maximum temperature of germination, which are specific to it, and between which it can germinate. We wrote a simplified article on when to plant out here.
Each of them also has an optimal temperature (which can vary according to the growing conditions.) The closer the soil temperature is to this optimum, the faster the emergence is, and the more the risk of rot or disease is reduced. So don’t be in too much of a hurry to release your seeds into the wild!
During germination, the ideal temperature is between 70 F and 75 F for most plants and between 77 F and 86 F for the Solanaceae family (such as tomato, pepper, eggplant, etc.). When the plants are up, the temperature should be around 65 F during the day and 59 F at night. However, do not think that there is no maximum temperature.
Even a demanding plant such as tomato, at temperatures above 86 F or at temperatures that remain high both day and night, has difficulty setting (the transformation from flower to fruit). The flowers dry up, or the fruits get damaged in consistency and color.
Temperature Requirements for Different Seedlings
| Optimal Germination Temperature | Seedling Type | After Germination |
| 54 to 60 F | Carrots, broad beans, lettuce, radishes, parsnips, peas, spinach | Keep cultivating at 50 to 68 F after germination |
| 60 to 68 F | Broccoli and cauliflower, fennel, chicory, parsley, chives, onions, kohlrabi, chard, leeks, beetroot, celery, savoy cabbage, corn and autumn beets | Keep cultivating at 60 to 68 F after germination |
| 71 to 78 F | Andean berries, pumpkin and zucchini, melons, aubergines, tomatoes, sweet corn, French beans and runner beans, cucumbers, bell peppers and peppers | Keep cultivating at 65 to 68 F after pricking out |
Soil Temperature Needed to Start Seeds
Each plant species has its own needs, so you can start sowing or planting outdoors when the soil has reached the right temperature for the species in question. In general, temperate climate plants require a minimum soil temperature of 54 to 59 F, while tropical origins require 64 to 68 F.
Eggplants, ground cherries, cucumbers, squash, zucchini, beans, melons, watermelons, peppers, arugula, potatoes, tomatoes, basil, etc., coriander, and summer savory. There are, however, three exceptions: carrots, Swiss chard, and cabbage, which, although they are temperate climate plants, prefer rather warm soil when planting. You can sow or plant the beets when the soil temperature has reached 54 to 59 F.
For many years gardeners have applied the rule that you can start planting when the night temperature is over 50 F for a period of 5 to 7 days. If this strategy is still valid, considering the ground temperature is essential.
Some lands heat up faster than others, and the presence of mulch can also delay warming. As for the respect of the lunar cycles or the references to saints to sow or to plant, we know today that these are not reliable methods.
To know the temperature of the soil, you should use a thermometer. You can use a short-stemmed thermometer or a compost thermometer with a longer stem. It has the advantage of meeting two uses. These instruments can be used both indoors and outdoors.
Why Is Soil Temperature So Critical?
We forget that a crucial part of culture’s success in plants is based on the root system. It is thanks to it that the plant is supplied with water and nutrients. If it is not in good health or the right conditions, that is to say; it cannot feed the plant; it will vegetate.
Ground temperature, particularly in species of tropical origin, is a factor limiting the absorption of water and nutrients. There is, therefore, no point in planting this type of plant in cold soil because, in these dire conditions, the metabolism of the plants works at a slower pace, or worse, it is stopped and takes time to start again.
When the temperature rises or falls beyond the conditions supported by the species in plants, physiological changes appear. These are different for each species and concerning the growth stage and the magnitude and duration of the temperature change.
Usually, it is respiration and photosynthesis that are disturbed. This heat stress causes growth to slow or even almost completely stop. When it is due to cold, the chances of it going into dormancy are greater than if high temperatures cause it. In this situation, it is often the flower and fruit setting processes that are affected.
To minimize physiological problems, the first thing to do is to respect the temperature needs of the roots. By doing so, you avoid the risk of heat stress and potentially too long dormancy.
How Do I Protect My Plants from Cold Nights?
On cold days and during winter, strong winds, low temperatures, and frost damage the leaves of plants. You should understand that temperature tolerance varies between different plant species.
To protect your plants from cold nights and keep them always beautiful, here are some tips:
Keep the Soil Wet
The soil will retain the heat captured during the day if it is moist, protecting the plant roots from the cold at night. Remember never to water the soil if it is frozen.
Water in the Morning
You should ensure your plants are well watered. Avoid watering the plants in the afternoon, as there will not be enough time and heat to dry before nighttime, harming the plant. But remember only to wet the soil, leaving no water on the leaves.
Cover Garden Plants
If you have frost-sensitive plants in your garden, try to wrap them in cardboard, plastic, fabric, or newspaper in the late afternoon to prevent them from suffering too much from the cold at night.
Use Sheets or Protective Screens
To cover larger plants or areas such as vegetable gardens and gardens, use sheets, plastic, or protective screens on cold nights. When a cold night is announced, attenuate the cooling in the foliage of the plants. To protect it, you can install protection using canvas against the cold or old sheets.
Plastic should be avoided, which, even if small, can cause burns of the foliage in the event of frost. The canvases are installed at the end of the afternoon and are removed when the temperature exceeds 10 ° C.
Use Dry Mulch
Cover the soil with a layer of dry leaves and twigs to protect plant roots. Wondering if it’s okay to mulch a vegetable garden? We took a deeper look here.
Fertilizing
When the cold becomes less intense, especially from August, apply fertilization to strengthen and favor the sprouting of your plants and flowers. Check out which fertilizers to use in your vegetable garden in this post!
Lift the Pots
Moisture and cold from the soil will pass into the pots on the floor. Place them on top of a wooden or plastic frame, which will help keep them warm.
Decrease Watering
The water takes longer to evaporate during cold weather, so reduce the weekly rules for indoor and outdoor plants.
Place Pots Near Walls
Place outdoor potted plants close to a wall, between shrubs, or, if possible, under a porch roof.
Search the Vases
It is important to provide thermal insulation to protect plant roots, wrapping the pots with newspaper, plastic, or cardboard layers. Wrap tiny seedlings in parts of a plastic bottle to keep them warm
You can increase the soil’s heat in a vegetable garden to bring forward the sowing or planting dates. In the long term, repeated inputs of organic matter help to reduce temperature variations in the soil. In the short term, you can use blackout clothes.
These are black polyethylene canvases that are temporarily installed on the ground. Suppose their first use is to cut the light to prevent the presence of unwanted grasses, placed temporarily a few weeks before planting, by warming the soil. In that case, they activate the beneficial work of microorganisms.
There is no such thing as a perfect temperature for growing plants. Instead, the temperature is a variable value that you can change as you grow your plants. Each type of plant likes a different temperature.
The best you can do is to ensure the conditions are right for you to plant outside. Soil temperature is vital, so you should use your thermometer perfectly to ensure the conditions are suitable for the proper growth of your plants.
