Transpiration in Tomato Plants – water evaporates through a plant’s leaves and is replaced by water absorbed through a plant’s roots – in an unbroken flow.
Why this is important for our plants
- If moisture is unable to evaporate from leaves, a plant cannot absorb moisture and nutrients through its roots.
- A lack of transpiration (evaporation) is a common cause of Blossom End Rot – interruption to the calcium supply.
- High humidity in the greenhouse can reduce transpiration and cause BER.
- Low transpiration can cause nutrient deficiency and reduces growth rate.
Pro Tip: Air circulation (aeration) around leaves is important to prevent many problems – open the greenhouse windows when the air inside is humid.
It’s like a good sweat! (humans perspire, plants transpire)
Tomato plants lose moisture through their leaves (by evaporation) and absorb water through their roots in an unbroken flow of moisture upwards through a plants plumbing system – a bit like people sweat and replace water by drinking.
This helps a tomato plant in several ways …
Nutrients are delivered to a plant in water absorbed by its roots.
If there was no evaporation through the leaves, there would be no water absorbed by the roots. When there is no water flowing through a plant, there are no nutrients available for a plant to feed on.
Plants are cooled by water evaporation in hot weather and receive more nutrients for growth.
The hotter it is, the more moisture is lost through the leaves, so the greater need for frequent watering – especially hanging baskets and small containers.
Some tomato varieties have more leaves than others. The more leaf area, the more water a plant needs and uses.
Also, the more water that flows through a plants system, the greater the amount of nutrients available. Too much nutrient absorption when plants are absorbing a lot of water can cause over-feeding issues.
Pro Tip: In hot, dry weather when moisture loss (transpiration) is at its greatest, reduce feed if possible.
Plant pressure keeps plants with soft tissue upright
When evaporation continues but there is no water for the roots to absorb, pressure (turgidity) drops and plants wilt.
Tomato plants can absorb water through their leaves, just as they can lose it through their leaves, so the quickest way to revive a wilting plant is to mist it with water.
Because the holes in the leaves (called stomata) are mainly on the underside of each leaf, it’s a good idea to mist the underside of the leaves – spraying upwards. This applies to reviving plants and when applying a foliar feed such as a seaweed tonic.
More points to consider
- When it’s cold – Evaporation (transpiration) is reduced so plants receive less food.
- When it’s humid – Leaves lose less water in humid air than they do in dry air, so plants receive less food.
- When it’s cold and humid (sounds like every morning in my greenhouse in the early spring!) – plants receive no food!
- When it’s hot in the greenhouse and the air is damp because there is no airflow, plants get no food – open the greenhouse windows!
Try to keep condensation away with good air circulation in the greenhouse, and try to keep leaves dry if growing outside.