This week, a short little blog (yes, gardening work has resumed quite intensively) about the temperatures for sowing and the growth of vegetables.

The optimal temperature

A priori, this first concept is clear. It is simply the ideal temperature for a seed to germinate or for a plant to grow under the best conditions, thus without thermal stress.

I start with this concept because it allows for the introduction of the following two concepts, which are the vegetation zero and the maximum temperature.

The zero of vegetation

Just as each plant has an optimal temperature for growth, it also has a temperature below which it stops growing! This is the zero of vegetation..

When I say that it is not damaged, it is true in the short term, but over time the plant can experience several negative effects.​

• Growth delay
  The plant no longer absorbs water and nutrients efficiently, which leads to stunted growth, consequently delaying flowering and harvest. Therefore, planting "too" early may be a false good idea.
• Vulnerability
  Prolonged exposure to temperatures that are too low weakens the plant, making it more vulnerable later to pests and diseases.
• Deficiencies
  The difficulty in absorbing nutrients can lead to deficiencies in the plant, especially for young plants whose root systems are underdeveloped.
• Quality
  The taste quality of the production (tomatoes for example) can be altered.
• Premature rise
  Finally, stress due to cold can cause premature bolting (going to seed) (lettuce, radishes ...)

That is why there is an ideal date for sowing vegetables. Of course, you can sow earlier as long as you are equipped to gain a few degrees to go above the zero of vegetation (heated greenhouse, cold greenhouse, cloches, tunnel Nantais ...)

The maximum temperature

Now that we have talked about the minimum temperature, let's talk about the maximum temperature. The first observation is that it is just as bad for a vegetable to be too hot as it is to be too cold!

Once the maximum temperature is reached and exceeded, the following harmful effects may occur

• Growth delay
  As with the zero of vegetation, growth slows down significantly, mainly due to the respiration of the plant🤔
  Yes indeed! A plant breathes, and we call this cellular respiration. This respiration burns energy, glucose that is provided by photosynthesis. The problem is that at too high a temperature, the plant breathes faster and faster, thus requiring more and more glucose, while at the same time, photosynthesis slows down because, to simplify, the plant closes its stomata (the pores of the leaves) to avoid dehydrating too quickly. Consequently, there is an imbalance between the supply and the need for glucose, resulting in a slowdown in growth.
• Vulnerability
  Vegetables that experience thermal stress are then more susceptible to diseases and pests, not to mention that certain fungal diseases proliferate in high heat (especially if the humidity in the air is high).
• Déshydratation
  If the water supply is not sufficient, the plant dehydrates and wilts.
• Drop in production
  The pollen can become non-viable. This is the famous blossom drop of tomatoes. The flowers bloom, turn yellow, and finally fall off without producing any fruit.
• Sunburns
  Like you, vegetables can get sunburned, which results in necrosis on the leaves and fruits.

In Belgium, our overall climate is cool and humid, and it is often in greenhouses that we face heat problems. To combat the heat, we can provide shade (shade cloth, tall plants that provide shade), ensure good ventilation, water regularly and at the right time (in the morning, late in the evening, at the base of the plants), and mulch to insulate the soil and reduce evaporation.

In conclusion, we must anticipate

To avoid problems, it is essential to anticipate as much as possible, and this starts with checking the weather to take the right measures to counter the effects of "extreme" temperatures, in either direction. For example, spreading mulch if the weather is forecasted to be freezing for several days or opening the doors and windows of the greenhouse several days before a heatwave. 

Everyone, depending on their environment, will implement the solutions best suited to their "local area."

In order to anticipate, here is the link to a table which shows the indicative temperatures germination and growth for a whole range of vegetables. I will add vegetables as we go. Feel free to let us know in the comments if you want us to add a specific vegetable to the table.

In the meantime, happy gardening and see you next time 😘