Good Gardeners Know How to Read A Plant. They Can Look At a Plant & Can Easily Diagnose Plant Problems.

I was taught How to Read A Plant many years ago. It was an essential skill on my market garden. But it is one that is hard to explain and teach others except when the crop or plant can be thoroughly examined. Trying to do it from photos is much harder, but in this article I’ll do my best to demonstrate what eventually becomes an intuitive skill in highly skilled plant growers.

Let’s start with a very simple plant problem that I often see reported online.


Etiolation is the Latin word that means the plant is drawn and looking for light. Not only do the stems grow very long but the plant is often pale and sick looking. This improbably the easiest plant problem to diagnose. But people often get it wrong.

The reason people get it wrong is that they think the plant has had plenty of light. But they judge it on what they think is plenty of light NOT what the plant needs. The thing is our eyes adjust to light levels. Our iris contracts if it is very bright and expands if it is darker. We don’t notice the extent to which this happens but it is quite a large response. It’s a bit like cats eyes and is the reason they can see in very poor light conditions. They are just better at it than humans.

Plants can’t adjust and have to put up with what they can get. And the light level in a window is often much worse than humans think, especially if it doesn’t face south.

How To Read A Plant: Tomatoes

I’m going to start with an example that many readers will be familiar with; tomatoes.

Look at this first picture. It’s a rather sick looking specimen that I’ve purposely mistreated to be able to show you how to read plants.

Q. What is the first thing that stands out?

A. It has two stems.

That’s unusual. Most tomatoes have a single stem. This is a growth abnormality where the plant has a stem that has split and it how has two stems. Don’t confuse this with a side shoot that has started to grew alongside the main stem. It is different and is a genetic issue. The closest thing I can compare it with in humans are identical twins where a single egg splits into two soon after fertilisation. Admittedly that is an imperfect explanation, but it’s the best I can do.

Double stemmed tomatoes are a mutation. They are not normal. However in most cases where I’ve seen it I have planted to plant and seen quite normal growth thereafter. I can’t however say that this would always be the case.

And in case you are wondering, NO you don’t get twice the yield of fruit. You might get an increased yield but not by a lot as the root is the same size and in one plant and the leaf cover doesn’t compensate enough to double the yield.

What else is wrong with the plant? What else can we read from the plant?

To make life easier, look at the next plant. One without a mutation. It shows the same problems as the first but without the distraction of being a mutant tomato!

What is the first thing that you see when looking at this plant?

Sadly there’s not just one thing wrong with it, there are several. And that is often an issue when caring for plants. Once a plant suffers in one way it tends to pick up other problems. For example poorly nourished plants tend to pick up pests and diseases far more. And plants infested with pests tend to get more diseases. Which comes first is likely thinking about the chicken and the egg.

The first problem I see with this plant are the yellowing leaves.

Then there are the necrotic(brown dying) patches on some leaves.

There’s also evidence of a problem in that the growing head of the plant is darker than the rest of the plant. Though not as dark as a healthy plant.

This is a very sick plant.

Plant Symptoms and Reasons For Them

The problem with looking at symptons is that they can be caused by a myriad of reasons. For example yellowing leaves can be due to nitrogen shortage, several minor nutrient deficiencies, under watering and pests.

To be able to fully read a plant we need more than just to be able to look. We need to know more about how the plant has been grown. The conditions it has experienced.

It’s a bit like going into hospital with a pain. The first thing they do is take a history. They want to know what lead up to the pain.

In the case of this plant I can tell you it has been in a small pot of compost that has dried out several times. Thats why it is showing many of the symptoms we can see.

Another problem is has suffered is insect attack. Look carefully at the stem and there are small pieces of white material. In this case they are the remains of aphid. Other symptoms often seen include sticky deposits which comes from the honeydew they excrete (it’s why ants love them).

Plant Colour As An Indicator of Problems

The colour of the plants leaves is a good indicator of health. For example this Stelitizia reginae has distinctly yellow leaves and is nitrogen deficient.

Another one, actually the clonal parent of the first plant, is much greener despite being grown in the same compost and room.

When I grew lettuce commercially this was something we looked for every day as we tended our crops. It became automatic and second nature. But there is a problem when doing it if inexperienced. In the case of lettuce the varieties are not the same uniform green colour. Some are very dark green whilst some are naturally a paler, almost yellow green. So we have to take this into account.

Interestingly, when selling lettuce to different markets we took this into account. Some populations prefer one colour shade over the other and wlil not buy those that “look wrong”.

Plant Responses to Temperature

Plants need sufficient warmth to grow, though each species has its own preferred temperature range. Plants such as melons, tomatoes, peppers, chillis and cues are all warmth lovers and don’t really get going until it is warm. Whereas crops such as lettuce can cope with cooler temperatures (but still cope when it is quite hot). Lettuce however can’t stand freezing once they are hearted as the tissues break down and diseases soon follows. They can however freeze in cold weather and not suffer any detrimental effects if they have NOT hearted. Commercially I used to plant in greenhouse during the winter and my plants would often freeze solid for weeks before thawing and growing to maturity. Freezing did them no harm, but you had one careful to get the soil moisture right .. too wet and they frost would cause damage to the plants.

Temperature has other effects on plant growth. For example, lettuce, at the rosette stage, when it is cold tend top hug the ground. Once it warms up a degree or two the leaves lift clear of the ground. Commercial growers know this is normal but this with less experience might well read this as being plants that are suffering in some way. They aren’t, it is normal.

Temperature along with day length also controls flowering and is often manipulated commercially to make crops flower. All Year Round (AYR) chrysants are a prime example. Lights are used to manipulate flowering. If flowering is retarded we need to both read the plant to see the lack of flower buds, but also look at the crop history to determine the reason. Of course, in a well run greenhouse, this should never be a problem. It does however demonstrate how severe factors can determine plant reactions.

A lack of temperature ie cold often influences growth as well. Indeed, cold is necessary for growth in many plants. Rhubarb needs frosting to give its best crops. Some seeds need stratifying (subjected to low temperatures) before they can germinate. and daffodils need a cold spell to initiate flower growth.

High temperatures are also sometimes necessary. Some seed can only grow if subjected to fire. It’s part of the natural process after a forest fire.

Several plants needs a differential between night and day temperatures to intimate optional growth. If they temperature fails to vary growth can be inhibited. Plants can also go dormant when temperature and day length changes. Deciduous trees for example lose their leaves and go dormant. Some plants go dormant when the reverse temperatures prevail and go dormant once it gets too hot or the day length is long.

Why do I explain all these variations. For one simple reason, to demonstrate that being able to read a plant is complex and not always straight forward. It males me smile when someone sees a photo and gives an opinion based purely on the photo. Without a lot more info its invariably difficult to give an accurate answer.

Plant Dormancy

Plant dormancy is interesting. Trees remain dormant until spring when buds begin to form and then open. But not all do it at once. Those in sheltered spots tend to be the first, they are in warmer sheltered spots and benefit from the warmth irrespective of day length, Some species are slower than others though, for example, outside my window is a Catalpa bignoides, the Indian Bean Tree (which doesn’t have beans on it or come from India). It leafs up much later than other trees growing locally. A related species is Catalpa ova and it leafs up even later. It would be very easy to read these trees and decide they are dead simply because others trees are prospering and these have no leaves. That would be reading the plant wrong. Being able to read the plant is one thing, understanding what we see is another.

An extreme case of late growth is the wisteria. Once established it flowers in April or May where I live, and the leaves come after the flowers. But plant one in spring and it just sometimes sits there and looks dead. It can be early to mid June before there is any sign of growth. I’ve seen several wisteria dug up and thrown away as being dead because someone has read the plant incorrectly.

Spotting Dry Plants

When plants suffer water shortage they suffer from growth problems. They need water to grow. But different plant types need differing amounts of water. Examples would include, cacti, celery and watercress.

So we need to be able to read our plants and spot when they need watering.

Her’s the same plant before and after watering having been quite dry. In the first image the leaves are being held almost horizontally.

In the second image the leaves are more erect and firmer. There is a visible difference.

Reading Plant Tropisms

Earlier I mentioned etiolation. It’s when the plant gets drawn due to being pulled towards the light. It’s an example of phototropism.

Another tropism worth knowing about is Thigmotropism. that’s the way a plant grows or moves in response to touch. It’s what makes climbers twist around branch and what triggers carnivorous plants to catch insects. Both are forms of thigmotropism, though they work at different rates.

One tropism I saw as a commercial grower was where a crop of lettuce lifted its leaves higher than normal. They also had leaves that looked much darker than normal. Reading leaf colour is hard, different varieties have lighter to darker leaves. And I used to grow some varieties for northern markets and other varieties for southern markets as there was a preference .. a veritable north south divide! But I digress.

The problem was high soil conductivity after growing a crop of tomatoes (soil conductivity is jus tone of the soil tests that can be performed) Heavy feeding of tomatoes increases soil conductivity and lettuce are sensitive to it. Often they fail to grow at all. But in this case the conductivity wasn’t severe and it was like the plants were trying to get way from these soil conditions. And that points out another mistake. Don’t anthropomorphise about plants. They adopt behave like us. To understand them and read them accurately we need to put ourselves in their roots (shoes) and consider conditions from their perspective.

Reading Chemical Damage in Plants

OK, so you are organic and don’t allow those toxic chemicals near your plot. The problem is chemicals have a way of getting in just the same. If you are near farmland they could drift on the breeze when a sprayer is being used incorrectly. And if you buy-in compost it could contain aminopyralids or similar.

If the spray is a weedkiller it can disrupt the plants hormones and cause problems. Many weedkillers ar hormone based and kill plants this way. Typical symptoms are easy to read. The leaves and sometimes the stems twist and contort. Sometimes the plants goes yellowish. If it is in the growing tip only, it is possibly glyphosate. The image shows cleavers damaged by a hormone weedkiller.

I’ve only suffered chemical damage once. In a large greenhouse I had one 90 foot long bay being used to test a foliar feed. the trial was run by a trails manager for the manufacturer. The other three bays were untreated. One morning the plants looked a little strange but I couldn’t determine what it was that was wrong. The next day the leaves were distorting much more and the damage obvious.

It seems someone had used the the sprayer to spray lawn weeds and hadn’t cleaned the sprayer out properly! Even in very dilute form the minute amounts of side had affected my lettuce. An insurance claim followed and they paid for all the lettuce in the greenhouse as we couldn’t be sure ghat they weren’t all contaminated even though no damage could be seen in the untreated areas.

Image: Stefan.lefnaer, CC BY-SA 4.0, via Wikimedia Commons

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