Do plants need water?
Faced with low rainfall and high temperatures ....
Faced with low rainfall and high temperatures there are lots of hints and life hacks been promoted all over the place. Put a brick in your toilet cistern, irrigate the garden with water from your bathtub, take showers not baths (oops) don’t wash your car or bicycle and on and on. Personally I feel that the best way to save water is for everyone to shave their heads. This means that people can take much shorter showers as they don’t need to wash their hair. Job done, smile.
We have all probably noticed that rain falls from the sky but the role played by the soil and the vegetation is a part of the water cycle which is, for most people, ignored. This is unfortunate, especially when it concerns farmers. So :
The bit we learn about in school. Water evaporates and condenses as clouds and then it falls as rain (or snow or hail). A lot of this rain falls on the oceans, some on land where we have infiltration into the soil and the water table, runoff, evaporation from the land/water bodies and transpiration from the vegetation.
The other bit. Water is ‘held’ in the the pore spaces in soils, vegetation accesses this water and it passes through them to rejoin the atmosphere. Bacteria from leaves, principally from tree leaves are carried into the air and become condensation or freezing nuclei around which water can condense and form raindrops or hail. Forests tend to have a higher levels of cloud cover than surrounding cleared land. I am talking here about temperate and boreal forests, for other types of forest the opposite can be true (see here **)
Yes I know that I have really really simplified the whole thing but the point is that we can either see rain as something falling from the sky or as vapour that is rising up from the land and vegetation. A change in a point of view can lead us to different insights.
Ok so back to the title “Do plants need water?” This is a question I often ask of clients and students and of course it sounds stupid. But there is a point, plants that need water are aquatic plants, the others need humidity in the soil. So obviously what I am trying to encourage is a change of point of view, basically to see the soil as a zone where water is ‘stocked’ in the form of humidity. It is also an attempt to encourage people to think soil humidity first and irrigation afterwards. This change of viewpoint can also help people understand that the humidity in soils isn’t chemically the same as the rain from which it comes. The rain seeps down through the soil layers and becomes charged with a wide range of organic molecules which are useful to plants. Permaculture designers look towards the soil first when considering water management.
A hierarchy of intervention.
Switch to agroforestry and sylvo-agro-pasturalist systems. Planting forests, in many areas, can increase local rainfall from 5 to 10% but this doesn’t help a lot if most of the rain runs off and goes into water courses and heads back to the sea, or inland lakes. It does however encourage us to move towards agroforestry systems rather than the open plain systems we see too much.
Increase the capacity of our soils to hold moisture. Stop ploughing! biochar, increase organic matter soil content etc….
Increase runoff infiltration into soils and water tables. Swales, half moons, gabion barriers, cease any activity that compacts soils ….
Reduce evaporation from our soils. Seeding direct into the remains of the previous harvest, growing main crops in soil cover plants, mulching …
Encourage a well developed fungal mycorrhizal network (see here *** research that is worth reading!!)
Stock excess runoff in reservoirs and tanks/cisterns.
Reuse gray water back into the system at both a domestic level and also at a community level.
Divert runoff from artificial surfaces to soils or water storage systems.
The rainfall average for the UK in 2021 was 1,239 millimeters, 1 millimetre of rainfall represents 1 litre of water per square metre. The total quantity of rain in 2021 in the UK would be a volume of around 308 cubic kilometres.
The data above hides seasonal variations and this is where some problems lie. When I was at school I was taught that there are 4 seasons, which is of course a sad reflection on the curriculum. Some zones have 4 seasons (at the moment) and plenty of zones do not or they are completely different. For example a climatic zone may have a long rainy season followed by a long dry season followed by a short rainy season followed by a short dry spell. I encourage people to think less in terms of winter/spring/summer/autumn and more in terms of dry/hot, wet/warm, dry/cold and wet/cold etc seasons. I have lost count of the number of times that a food grower has told me that their main problem is a lack of water or that there isn’t enough rainfall. Yet when we looked at the rainfall data for their area we often find that there is sufficient rainfall but most of it has run off.
As gardeners and farmers we have to work out the best water strategies for where we are. We have to take into consideration the soil types, the seasons, the average rainfall, the shape of the landscape specific to where we are. A part of this analysis will be calculating the quantity of water we need to store to see us through the increasingly long and dry periods. We need to know how many rain butts, reservoirs, micro-dams etc we will need. That said the other strategies 1 – 4 and 7,8 in the hierarchy are used to reduce irrigation needs.
Three final points, most of our crop plants don’t need full sun, they grow and produce perfectly well, if not better, in semi shade. Another point is that a lot of irrigation is excessive. Plants that have to struggle, a bit, root deeper and produce fruit that have a better flavour. Finally it is imperative that we start to cultivate plants that are adapted to the hotter drier conditions we are facing. We cannot continue to try and force plants to grow in areas where they are no longer adapted just because we grew up eating them.
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