Plants Need Water. It’s Essential. But Many Gardeners Find Knowing When To Water A Garden a Mystery. This Article Strips Out the Mystery, Myth And Magic. Here Are My Watering Secrets.
In this article I look at watering basics. Starting at why plants need water I then answer your questions on the following How to Water Your Garden topics.
- Why Plants Need Water
- How Often Plants Need Watering
- How Much Water Plants Need
- How To Reduce The Need to Water
- How Plants Reduce Their Need for Water
- When to Water Plants
- How To Water Plants
- Watering & Irrigation Equipment (from watering cans to automated systems).
- Sustainable Watering (from economic watering to how to save water)
- How Farmers Water Vegetables (from Soil Moisture Deficit [SMD] to Impact on Yields)
- How Gardens & Gardeners Waste Water
- Drought Resistant Gardening
- Drought Resistant Plants
- AND Much More
Why Plants Need Water
Between 80-95% of a plant is water. So water is essential to plants. They need water to help them photosynthesise, to move minerals and nutrients around the plant and to help cool them in hot weather. Some plants will survive on less water than others and survive desert conditions. Other plants actually live in water. In both cases the plants have adapted to live in those conditions and struggle to live in less favourable conditions.
The plants we grow in our homes and gardens tend to occur in two main groups. Those that are quite drought resistant and those that need a lot of water. At one extreme we have the xerophytes. Xerophytes are the plants adapted to dry conditions to why there is little liquid water.
Cacti are typical desert plants. But it’s not just a lack of water that matters, It’s water accessibility. In polar regions and high in some mountain ranges there is often plenty of water. But it’s frozen and the plants can’t get at it. It might as well be a desert.
At the other extreme are plants such as rice that grows in water for at least part of its life. And whilst most of us will not try rice growing in the UK (for rice to crop it needs high night time temperatures for many weeks), we might try to grow celery. Celery is a plant that, in the wild, grows in marshy places. It needs loads of water. When I grow vegetables and salads commercially it was the crop that used most water. Even when it rained we often put the irrigation on to give it even more.
Before moving on to the next section about How To Water Your Garden & Plants, its worth thinking about other reasons plants struggle to find enough water. In some cases plants grow in high salt situations and experience a surfeit of salt. In these cases they will have developed a physiological process to deal with the salt, but getting water is much harder for them. However, many of them now need salt to survive, eg samphire.
How Often Do Plants Need Watering & How Much Water Do Plants Need?
And when should you Water Your Garden?
The amount of water a plant consumes largely dictates how often we need to water it. For example if the weather is hot, sunny and/or windy then it will transpire more and hence need more water. If it is growing fast it will need more water, and that includes when it is producing fruit, tubers etc.
Conversely, in cold weather most plants need less water. Indeed, in winter, I used to water crops such as lettuce being grown in greenhouses very infrequently. Too much water was likely to cause them damage. Especially if they froze (I grew a lot of unheated lettuce crops).
Where the plant is growing also affects the amount of water they need. Plants in the ground generally need less than in terracotta pots simply because terracotta pots are porous and dry out quicker. This is less pronounced in plastic pots but the plant will still need watering much more than if the same plant were in the ground.
The same principle applies with raised beds. They drain faster than the ground levels beds. This is mainly because they are more exposed to sun and drying winds. It’s one of the reasons I dislike raised beds (though as I get older I appreciate it is harder to reach my toes).
One of the worse ways to grow plants, in my opinion, is in grow bags. I know some people will disagree, but they have limited volume and dry out fast.
The size of the plant in a limited volume of soil or compost is another factor to be taken into account. Bigger plants consume more water. It’s that simple.
So, with that explanation out of the way let me answer the question.
How often do plants need watering and how often should we water the garden?
The answer depends entirely on all the factors above. A heavy crop of tomatoes in a greenhouse needs a lot of water EVERY day. My market garden was on a free draining soil, and though we added a lot of organic matter, the crops needed around half a gallon of water a day in summer. Earlier in the season, when small, they needed perhaps only two pints a day, but this increased as they grew. Even on wet days we watered them quite liberally. If the weather was wet for several days I decreased the amount of water a little bit but not much. If it was warm they still grew fast.
Crops such as celery were given 1-2 hours of irrigation per day outside and at least 20 minutes if grown in greenhouses. This didn’t decrease much if the weather was wet and overcast as they need a lot of water.
Lettuce inside were generally watered every 2-5 days in mid summer but it depended on how dry the soil was and how the crop looked.
Outside lettuce crops were irrigated when they needed it. I watered as infrequently as possible, but lettuce respond very well to watering and if watered late evening the rate of growth was visible in the next 24 hours. Watering was therefore used to control the rate of growth and harvest date consistent with our orders. Bank holidays meant sales whilst the days afterwards always saw decreased sales. A wet weekend saw sales plummet, but promise of a sunny bank holiday saw orders go through the roof!
I previously commented about how a crop looked. Crops tell us when they need watering. Once you get experience its easy to look at a field of lettuce or a greenhouse full of tomatoes and know what they need. Lettuce will show subtle signs of wilting days before they actually get under stress. And if you slightly overwater a tomato there will be small droplets of water on the leaf margins next morning. This soon evaporates so its important to look early in the day. But when you look later it’s possible to see a slight lowering of the leaves as they look for water. If they get to the stage where they are clearly wilting then it’s far too late. They needed water before getting to the stage.
Water Your Garden: Little And Often v Infrequent Watering?
Bearing the above in mind you might begin to see the watering isn’t easy. It’s one of the most skilled jobs in the garden. But to make it a bit easier here is a simple rule that applies to most situations. Don’t water little and often.
If you give a small quantity of water it doesn’t go far into the soil or compost. This results in the plant growing its roots where the water is found. It doesn’t bother to grow deep roots. This means it has no resilience when there is insufficient water.
When water is given in volume but less frequently, as the soil begins to dry from the surface the plant will drive its roots deep to find the water that penetrated deep. By doing so it becomes more resilient and less likely to be damaged by drought. It means that you can choose when to Water Your Garden rather than be forced to water because plants are wilting.
How To Reduce The Need to Water
It’s easier too reduce the need for watering than many people think. And with watering being time consuming and water being expensive that’s a good thing.
Let’s go back to basics again.
I’ve already explained that plants consume more water on hot, sunny and windy days. So the secret has to be to reduce these factors. I don’t like the idea of shading plants too much (except shade loving plants), but we can reduce wind and heat.
In the greenhouse we can ventilate or poor water on the floor to reduce heat. I know pouring water on the floor sounds counter-intuitive but the cooling effect reduces water consumption a significant amount and reduces plant stress. So it’s a double whammy of success when you do.
And sheltering from the wind has a huge effect as well. In the flat fenland fields of East Anglia you’ll often see boundary shelter belts comprising tall poplar trees. they reduce wind speed and that reduces water consumption. One of the techniques that proponents of Dry Gardening use is the modified shelter belt. Instead of tall trees they often erect small windbreaks such as a piece of trellis. This not only provides something for plants to climb up but also slows the wind down. This reduces evaporation.
Another trick to reduce water usage is to produce a dust mulch. It works like this. Between row crops the soil is hoed on dry days. This produces dust which water has a problem penetrating from below. Hence the surface is “insulated” from the drying sun and less water is loss from the bare soil. The down side on a large scale is the wind creating a dust storm. That is a problem on the prairie like Fens, but not likely to be in an enclosed garden. Here it works really well.
Lawns soon go dry when the soil dries out in summer. The result is brown patches or worse, dead patches. The problem is caused by the sun beating down on short grass as this leads to high evaporation rates from the soil surface. This is significantly reduced if the grass is thicker. The answer is therefore not to cut grass as short. Long grass shades the soil, this leads to less evaporation and less need to water.
Choosing the right crops
Growing the right crop also helps reduce water consumption. We know that crops such as tomatoes and the cucurbits (cues, courgettes, melons etc.) consume huge quantities of water. So the answer is to grow fewer of them. OR grow them in a more moisture retentive soil. Increasing the soils capacity to hold moisture is achieved by adding organic matter. In fact, at the extreme you could grow cues, melons etc on top of ugh compost heap. They benefit from the warmth of the rotting organic material, the nutrients and moisture.
Some crops will grow quite well with little water until a certain stage of growth after which the often benefit from irrigation. For example farmers rarely water potatoes in the early stages but do irrigate once the tubers start to swell. They know that watering art this stage is the best time to do it, and that earlier watering has little impact on yield. Of course, most of us don’t water potatoes, in the garden the benefits are not that significant.
No Dig: A way not to Water Your Garden?
No Dig gardening has an edge when it comes to watering. The reason is simple. Every time we dig or rotate the soil we encourage evaporation of the soil moisture. It dries out quite rapidly if cultivated on drying days. Because No Dig doesn’t turn the soil then evaporation is eliminated.
There’s a second reason the No Dig also wins in the watering game. No Dig encourages a better soil structure and the formation of fungal networks in the soil. The soil structure encourages the plants to root deeper and allows water to rise through capillary action. Plus the fungal networks move water around the soil and provide the plants they have a symbiotic relationship with to benefit from their access to water. That’s something that is lost when No Dig isn’t practiced.
How Plants Reduce Their Need for Water
Plants take in water through their roots and lose it through their leaves. They have to maintain a balance between the two. And when the temperatures are high and the wind is blowing that’s hard. So they reduce the losses by reducing the losses through their leaves. The leaves naturally lose water through little valve-like leaf structures called stomata. And the plant can close their stomata when need be. This reduces water loss and stops them wilting quite as much.
Some plants have modified their leaves to reduce evaporation. And some plants shed their leaves in hot summers to reduce their need for water. Plants are incredibly resourceful in so many ways.
In Namibia’s coastal deserts, along the infamous Skeleton Coast, there is no rain most years. So some plants have modified their structure to condense and collect coastal mists. The mists condense on the leaves and are the droplets are funnelled down to the roots. It sounds incredible, but when I visited this area a few years ago I saw it for myself. Seeing is believing.
When to Water Plants
Knowing that water is loss to plants when its hot, sunny and windy gives us plenty of clues about when to water. There is no point watering when evaporation rates are high. Eg when the sun is high in the sky and temperatures are at their fiercest. Or when the wind is blowing hard.
Commercially I watered sensitive crops such as lettuce when they “hearting up”. It’s the time when watering makes a significant difference. This meant using a self propelled sprinkler across the field and that took several hours. Done when it was hot and windy meant crop damage and high evaporation rates. But, if done at night, with no sun, it was more efficient. The other benefit is that once the sun goes down, the wind usually drops and the water doesn’t get blown off course!
Greenhouse plants such as lettuce were always irrigated in the early morning, before the sun got too hot. This also gave the crop time to dry off before dark and reduced fungal disease risks.
Tomatoes, cues and related crops were also irrigated in the mornings. In most cases it was with drip irrigation, but more on that later.
Pots are different. Research indicates they are best watered in the afternoon or early evening.
Watering & Irrigation Equipment (from watering cans to automated systems)
At the most basic a watering can is one of the best small scale watering systems I know. the spout directs water exactly where you want it and the rose (sprinkler head) provides a gentle shower of water over a larger area. Roses come in various hole sizes from very fine, suitable for watering seed trays without washing the seed away, to extremely coarse which is suitable for applying a deluge of water to the base of shrubs and trees.
In commercial greenhouses the chosen methods for watering crops are many but can be basically split into overhead sprinklers and drip irrigation. Drip irrigation is ideal for row crops such as tomatoes, and apply a given amount of water per hour. For tomatoes I used a drip system the provided two gallons per hour per plant. So a 15 minute watering gave each plant half a gallon of dilute feed and water and this could be varied depending on crop size, weather etc.
Overhead sprinklers produce volumes of raindrop like water that gently falls onto the crop. It’s as close to rain as we can get.
Both overhead and drip systems can be automated via computerised systems to irrigate the crops as often as needed 24/7.
Both overhead and drip systems are also ideal for amateur use.
Outside overhead sprinklers of various types are also available to commercial growers and amateurs.
At there most basic level the standard lawn sprinkler sits on the soil and send an oscillating jet of water into the air. It then breaks up into drops as it falls to the ground.
Giant irrigation systems
Farmers and commercial growers use systems similar to lawn sprinklers . The largest are rain guns that send huge jets of water into the air. These then break the jet into droplets in the same was as lawn sprinklers work.
The systems are usually self propelled, with the water jet mechanism fitted on a wheeled frame that is slowly pulled through the crop. I used a small version of this that irrigated a 80 foot swathe of my field with 100 gallons of water per hour. The big ones can consume thousands of gallons of water per hour. Without them we’d not see so many veg in the shops! The price is high however. It’s the environmental cost of water abstraction from rivers and aquifers.
A garden system I frequently see suggested is the seep hose. It’s a porous hose that is laid around the beds and along row crops where the water seeps through the hose wall. It sounds good theory but is affected by water pressure and slopes. Gravity and water pressure affects the even distribution of the water. Personally I think they waste a lot of water due to this.
Sustainable Watering (from economic watering to how to save water)
I’ve already mentioned how watering at the right time of day, when evaporation is minimal reduces water usage. And that giving plants an occasional good drench is better than little and often (tomes, cues, celery etc still need daily watering).
Mulching for moisture retention
Mulching plants also saves water as it prevents so much evaporation. Plus, organic mulches increase the moisture retention qualities of your soil or compost. I much prefer organic mulches but there are occasions where inorganic mulches work short term. For example an inorganic collar round the base of a tree will reduce weed competition and hence water usage. An organic mulch will be better though.
Native and drought resistant plants
If you choose native and drought resistant plants you will not need to water as frequently if at all.
Beth Chatto’s book, The Dry Garden, is an inspiration on the choice of plants that will survive and thrive without watering. Her East Anglian garden had less rainfall than Jerusalem and thrived on it.
Plant choice affects drought tolerance
If we grow celery then we need to water every day. But that’s not true of beetroot. They go ages without rain or irrigation. In fact they would only need watering in the most severe of droughts. And then I’d question whether it’s worth doing. Sometimes the way to save water is not to water at all.
Don’t mollycoddle vegetables
I often see online posts asking how often cabbages, carrots etc need watering. The advice, from amateurs, is often to water every few days if it’s not raining. But it really isn’t necessary.
When I farmed we’d plant large numbers of cabbage, sprouts etc. This was often in hot weather. We didn’t water them in.
What we did do was transplant them into soil that hadn’t been recently cultivated or ploughed. That way it had a lot of retained moisture. We then planted well toughened plants deep as we could. This pushed the roots deep into the soil where they could find moisture and start to grow their roots even deeper. The plants were grown “hard” ie not watered too often. This way they were toughened to expect hard conditions. We did however give them a good watering 2-3 days before lifting them for transplanting. If we had to store the plants a few days between lifting and transplanting we’d keep them in a dark, cool place covered with a damp hessian sack.
Once planted the brassicas often suffered a bit. Sometimes they lost leaves. Sometimes they wilted badly. It was tempting to irrigate to “save” them.
Avoid irrigation at this stage.
Inexperienced people, looking at the plants, declared them dead. But, provided the “cocks”, the plants growing tip stayed green the plants always recovered.
Reading this you possibly question why I didn’t water them. There are several reasons. Watering at this scale is expensive. Watering is extremely time consuming unless you have expensive equipment (and my irrigation was better used on lettuce crops that made more profit!). The plants also grow “soft” and rarely yield as well, these reasons are the most important.
Weeds compete with crops for moisture, light and nutrients. So if you eliminate weeds you improve the situation for your plants in so many ways. Hoe the weeds when small or use mulch to suppress them. If fact do anything to get rid of weeds that compete for water.
I’ve written extensively on the harvesting and storage of rainwater, follow the link for more on this. But it doesn’t stop there. Many gardeners harvest grey water. Thats water that has been used for cooking, hand washing etc. It’s not a good idea to use water that is full of chemical cleaners for this purpose but water with just a little soap is fine in the flower garden.
My wife and I spent a few months in Cape Town a few years ago. It was at a time when water was being rationed. Baths were banned (in hotels they removed the plugs), car washes closed, and each person had a nominal water allowance per day. Each day the news was about water levels in the reservoirs. We visited some of them, they were just dry mud for as far as we could see. It made us very aware of how precious water is. And when I came home to the UK I installed rain water harvesting equipment.
Avoid overwatering your garden
Most plants survive a long time between rain events. Evolution has conditioned them to do so. In nature no one waters plants, only the weather looks after them.
So think twice before watering the garden. Pots certainly dry out quickly, but deep rooted plants will survive and even continue growing for a long time between rain events.
And when you do water, give them a long drink that goes deep into the soil and makes them root deep for it.
How Farmers Water Vegetables (from Soil Moisture Deficit [SMD] to Impact on Yields)
Farmers and growers do sometimes irrigate crops. they do it when there is a measurable yield or quality increase or where it makes economics sense eg to space crops out for supply contracts with supermarkets.
Crops such as potatoes show significant yield or harvest date improvements after irrigation. And getting early potatoes to market a few days earlier can make a huge difference to revenues. For gardeners those few days rarely matter.
Farmers decide when to water based on observation of the crop and measurement of Soil Moisture Deficit (SMD). SMD is basically a measure of the water applied to a crop by irrigation or rain against the water used at that stage of growth. Different soils have differing water holding capacity, called its field capacity, and differing crops have differing tolerances/preferences as to how close to field capacity they best grow. Ideal deficits are around 10-27 mm depending omen the soil type, crop stage and variety.
SMD also get used to control certain potato diseases and physiological disorder. Farming is extremely science based these days and work on water tolerances as low as a few mm.
When it comes to gardening, my advice is “don’t try this at home”! It’s not possible. But it does give an inkling as to how food is produced on farms for consumption by the majority of us.
How Gardens & Gardeners Waste Water
Gardeners waste water by not harvesting and storing it AND by not following most of the advice above. Follow the advice and life becomes easier.