Tomato plants need water, light, the right temperatures and nutrients (such as tomato food) for good growth. If one of these is in short supply – the limiting factor – the factor that restricts growth.
For example, it doesn’t matter how much water, light and food we give them, if they don’t have the right temperature range in which to grow, progress will be slow or there may be no growth at all!
The Weakest Link
When we assess the progress of our plants, we should consider the limiting factors to see if there is one that may be holding growth back – a weak link in the chain, so to speak.
- In the early part of the season, low temperatures and low light levels may be the limiting factors.
- In the summer, too little or too much water, high temperatures and nutrients (over or under feeding) may be the limiting factors.
The idea is to be able to assess our plants at various points of the season and spot whatever is in short supply.
Sometimes there is nothing we can do about it, the weather for example, but there are many times when a slight adjustment can make all the difference.
- Using shade cloth (garden fleece) to keep temperatures down
- Using white reflective surfaces to increase light levels in the polytunnel and greenhouse
- Watering in the morning when night temperatures are low so that plant root won’t be sat in cold water overnight
- Pruning leaves to avoid plants becoming too vegetative
- Using correct watering methods to avoid soil saturation and damaged roots
The list goes on …
When The Weather Is Hot
Keeping plants correctly watered, without over or under watering them is a huge challenge. A watering valve or reservoir is the best way to deal with the situation but even these methods can struggle when providing for large fruiting plants.
I’ve had a number of emails recently about blossom end rot owing to the high demand by plants on their water supply.
When temperatures reach around 35 C, which they can easily reach in a greenhouse in the midday sun, photosynthesis slows or stops altogether and plants run out of energy.
Shading plants with cool glass or fleece, using white reflective surfaces on the front of black pots and misting with water morning and evening can help reduce temperatures. Misting also reduces the rate of transpiration and helps plants conserve water – albeit temporarily.
Nutrients and Water
Giving the right amount of nutrients in hot conditions is also a challenge.
Large fruiting plants need a lot of water but if that water contains a high level of nutrient salts (from the soil or from the watering can), the rate of water intake by the roots is reduced (see osmosis). This can have the affect of plants wilting in hot weather as they struggle to absorb water quickly enough to cope with the high rate of moisture evaporation from their leaves (see transpiration).
The tip here is not to over feed in hot weather, in fact, reducing the amount of mineral salts (such as Tomorite) will be helpful for our plants. They will then be able to absorb water more quickly.
If you feed using organic methods, you won’t have too worry about mineral salts slowing the rate of water intake because organic food contains much less salts than Tomorite for example.
Leaf Pruning (please read carefully before chopping!)
It’s a good idea to remove leaf branches up to the first truss – when the first truss has started to set fruit. However, it is important to understand that the closest leaf branch to each truss of fruit, provides the tomatoes on that truss with most of the energy they need for their growth.
In other words, the leaves closest to each truss of fruit provide most of the energy for fruit growth.
This is very important to keep in mind when we are removing leaf branches. For example, if there are three leaf branches between the first and second trusses, the best way to prune is to remove the middle leaf branch, leaving the branches nearest the trusses.
Removing all of the leaf branches up to the second truss can be done when the fruit on the second truss is ripening (not just setting). In other words, leaf branches can be removed up to the truss that is producing ripe fruit.
If you are only growing four or five trusses, I wouldn’t worry too much about removing branches up to the ripening truss – just remove leaf branches up to the first truss, middle leaf branches and trim long leaf branches if necessary.
You may wonder why we need to prune at all!
It’s because we need to keep a balance of leaves/fruit and keep a plant producing enough energy from the leaves to supply to the fruit.
Tomato Plants Have Two Phases of Growth
I sometimes refer to three phases, but technically, the flowering and fruiting stages are one phase.
- The vegetative phase – producing the stems and leaves etc.
- The generative/reproductive phase – producing the flowers and fruit etc.
A tomato plant begins its life in the vegetative phase and when flowering and fruiting begins, it moves into its reproductive phase.
However, these phases are not mutually exclusive – in the second phase of flowering and fruiting, we still want plants to be growing stems and leaves to support higher trusses. The main objective is to provide enough energy for the continued growth of a plant and its tomatoes, over a given period of time. This means that a balance of vegetative and reproductive growth is required.
This is where pruning comes in … also known as plant steering.
We want to direct energy from the leaves to the fruit. We don’t want to create a lot of big luxurious fat juicy leaves that when they get a bit older will become a drain on the plant’s resources and reduce the amount of tomatoes we get! By the way, tomato leaves are poisonous – don’t eat them!
Check Your Trusses
One way to tell if your plants are too vegetative (apart from having lots of leaves) is by the position of the trusses.
If your flower trusses are pointing upwards, the plant is probably too vegetative. If a truss is roughly at 90 degrees, that shows a balanced growth.
Leaves In Shade Are Less Important
For example, if you are growing plants up against a wall or fence, the leaves at the back will be producing much less energy for the plant than those at the front in direct sunlight, so shaded leaves can be removed (as long as there are enough leaves at the front of course!).
If you’ve ever tried to grow large tomato varieties that run out of steam at the second or third truss, it’s probably because the vegetative and reproductive balance needs adjusting. These adjustments can also be made by the amount and the balance of nutrients we give our plants.
Blossom End Rot
Owing to the hot weather that many of us have experienced lately, Blossom End Rot has become more common than usual.
Keeping a regular water supply to the plants is one of the biggest challenges we face, especially in hot weather.
Spraying when fruit are swelling with Chempak Calcium definitely helps avoid BER.
Milk is also an alternative – use skimmed milk as a foliar spray at 50/50 water/milk on the leaves. Best avoid spraying the fruit because fatty milk can cause a skin infection – also your greenhouse will smell like a dairy!
That’s it for another week – always good to hear from you!
The above article “The Limiting Factor In Tomato Plants” is from Nick’s Tomato Growing Newsletter.