Plants need water; that is a fact. However, to make the most of this, often limited, resource, it pays to use it economically by understanding plants’ needs and using techniques to help limit water loss from both plants and soil.
Firstly, we need to consider what plants need water for. Most of us remember the equation for photosynthesis from school days. Water is combined with carbon dioxide to make the necessary sugars which can then be converted into energy, the energy that is needed for growth.
Some of the plants will store this product of photosynthesis in readiness for future growth – such as in the swollen tap roots of carrots or beetroot or in the leaves of onions and cabbages. So, without efficient photosynthesis, we would not achieve good yields of our fruit and veg.
However, less than 5% of the water taken up by plants is actually used in the actual function of photosynthesis. The vast majority of the water is used to transport the various sugars and nutrients through the plant to where it is required before being lost through the stomata of the leaves by transpiration. This very important as if water is in short supply, the stomata will close, and photosynthesis will be reduced as carbon dioxide cannot get into the leaf.
So, our aim is to ensure the plants have enough water to maintain transpiration and, if they are crops, to put their growth on the parts we wish to harvest. This may sound obvious, but that is where efficient irrigation can pay dividends. You are selling the roots of the carrots, not the tops; the fruit of strawberries, not the foliage.
My top ten ways to achieve water efficiency in the garden are not just about how and when to water your crops. Reducing the amount of watering that is necessary in the first place will save you time and money.
Top ten tips for water efficiency in the garden
1. Plant selection
Think about your site, particularly in terms of soil type and drainage. Whilst you can ‘improve’ some aspects of your soil, as described below, it is better to think about the suitability of crops to make management, particularly their watering needs, a little easier.
A light, sandy soil will generally be well-drained and dry out quickly. On the plus side, it will be easy to work and warm up quickly in the spring. This will be great for fast-growing and early crops – so make the most of it for these. Light soil is ideal for ‘baby’ carrots and beet, legumes, salad crops, and early potatoes.
A heavier soil with a high clay content can be quite difficult to work – both when wet early in the season and when dry in the summer.
Adding organic matter will help this, as it will all soils. This is the best soil for perennial or deep-rooted crops, such as fruit trees and bushes, as the plant roots can manage to extract water even when it looks ‘bone dry.’ This is not to say other methods, such as mulching, will not help increase yields.
2. Soil Management
The roots are the most important part of any plant! It is the roots that absorb the soil water, so a good rooting zone enables them to access water (as well as nutrients) much better than a compacted or poorly structured soil and can often eliminate the need for extra watering or feeding altogether.
Cultivations, such as double digging to break up compaction or ‘pans’ (impervious layers in the soil), may be needed. On the other hand, if you have good soil, it may be worth considering the no-dig approach as moisture is lost every time you cultivate.
I simply cannot over-emphasize the benefits of adding organic matter to the soil. Garden compost, well-rotted farmyard manure, mushroom compost, leafmould, green manures, or any other bulky organic matter all break down to form humus – the most wonderful, almost magical, a substance your soil can contain.
Organic matter was proven to absorb many times its own weight in water, along with dissolved plant nutrients, which is then available for plant growth, having a buffering effect against drought and plant stresses. Digging in organic matter during cultivations helps retain extra moisture and can provide plants with the equivalent of an additional 2in of rain: about 20 days’ supply for many plants.
Clay soils are known for washing water away as it lacks the absorption properties of other types of soil. To fix this issue and make clay soil accept water more quickly, you will need to add some organic matter will. Organically improved sandy soil holds water longer, and as an added bonus, you don’t need to water it as frequently.
We are often very wasteful of water when it does rain, by allowing it to run off into drains or even causing erosion. Use diversion drains, swales, or terraces to help catch water flow and spread it out. By doing so, you will help water seep slowly into the ground where you want it rather than being lost.
Build low mounds around plants such as squashes so the water can be directed to their roots rather than spread out.
Soil is lost from the soil surface by evaporation and the plants themselves by transpiration. Both of these will be increased unnecessarily by wind blowing across the site. Shelter provided by hedges or non-living windbreaks, such as netting, will considerably reduce the need the irrigation.
On a hot day, up to 70% of water can evaporate from the soil on a hot – or windy – day. A good mulch is one of the best and probably easiest moisture-holding strategies you can employ. As well as preventing evaporation from the soil surface, mulch can help suppress water-thieving weeds from growing. Organic mulches, such as garden compost or manure, will also add vital nutrients to the soil.
It may not be feasible to put mulch around all vegetable crops but can really help hold the moisture if applied over potatoes or on the soil around zucchini and runner beans. Fruit trees and bushes really appreciate a good thick mulch of well-rotted farmyard manure around their bases.
Bare soil is the worst for losing moisture, so a layer of organic matter placed over bare soil will help reduce evaporation as well as adding nutrients to the soil as it breaks down. Alternatively, plant a green manure crop on bare soil, which can then be dug in to add organic matter before the next crop.
5. Control weeds
Weeds steal water! They can also swamp your crop and introduce pests and diseases.
Perennial weeds should be removed by digging or forking out during cultivations, whilst annual weeds can be hoed off on a dry day before they seed and left on the soil surface to die off.
You also need to be tough. Don’t waste water on unhealthy plants. Make the decision if a crop is not doing well for any reason. Remove and replace it.
6. Efficient Irrigation methods
Hoses and watering cans are still used by most of us, as we can water plants as and when they most need it – sowing, planting, transplanting, or as the fruits, roots, or tubers are developing. The watering should be aimed specifically at the stem bases beneath the foliage canopy, leaving the surrounding soil dry. This helps to limit weed problems and ensures all the water goes where it is needed.
Semi-permanent systems, such as seep hoses or ‘leaky pipes’ are ideal for plants in rows, particularly under cloches. These can be hidden beneath soil or mulch, which also avoids evaporation losses.
Automated irrigation systems, such as drip or trickle irrigation systems, will help save time and labor and are great for permanent, perennial plantings if set up from the start.
7. When to water, part 1 (stage of crop)
There are certain times when irrigation (extra watering) will be needed in order to ensure success with your fruit and vegetable production.
Firstly, when sowing seeds. Seeds are about 5% water yet soon germinate to a plant which is of 95% water! So, without sufficient moisture, these seeds may fail to germinate, or the resulting seedling will be weak.
Larger seeds, such as beans, can be pre-soaked overnight before sowing. You can see them double in size – so just think how much water that would have taken from the soil.
Drills for smaller seeds, such as onions or carrots, can be pre-watered before you sow the seeds. This has the advantage of ensuring the soil around the seeds is moist without encouraging the germination of weed seeds on the surface.
Secondly, when planting out or transplanting seedlings or young plants. Leeks or cabbages that have been sown in the nursery bed will require watering the day before transplanting and then watered in when moved. Likewise, plants that have been raised in pots or modules need to be moist when planted out.
Thirdly, when planting out young fruit trees or bushes. Even when planted in the dormant season, thorough watering will settle the soil around the roots and prevent dehydration. A watering tube can be put in when planting fruit trees to ensure water is directed to the roots, and watering may well be required for the first full year after planting in a dry year.
Finally, the most efficient time to apply extra water to most crops is at the stage when the part you wish to harvest is being produced. To water potatoes or root crops too early will result in lots of top growth, so the time to water is when the roots or tubers are swelling.
Although they do not want to be under stress, never overwater peas, beans, or tomatoes in the early stages – you want the fruits, not abundant foliage. Yields of soft fruit and the storage ability of top fruit will be improved if watered as the fruit is swelling.
8. When to water, part 2 (depth and time of day)
Plants need to be watered correctly. Generally, there should be enough to wet the top 15 to 30cm of soil, depending on the rooting depth of the crop, keeping in mind that too little water will just wet the soil surface and it does not reach the roots. Another disadvantage of light watering is the encouragement of roots to grow upwards, where they will eventually dry out.
On the other hand, too much water will drain out of reach of the roots or waterlog the soil, causing root rot. Dig down near the plant to see how far the water has reached – better a good watering every few days than insufficient every day.
Generally, the optimal time for watering is early morning, before the temperatures rise, winds are generally lower, and there is less evaporation. Morning watering is recommended since it gives the plants a good supply of water to face the heat of the day.
Normally avoid evening watering, especially on the foliage early or late in the year, as this can encourage fungal diseases.
9. Collecting, storing, and re-using rainwater
If your state allows it, install water tanks rather than waste the rain that does fall! We all have lots of roof run-off from our houses, sheds, and greenhouses, and this can all be saved for use on our crops. Water storage containers should be well-secured and covered both for safety reasons and to prevent the growth of algae.
Water from tanks is best used on the ground around plants rather than on their foliage, particularly on plants that are close to harvest and those of which we eat the leaves, such as lettuces.
10. Avoid overwatering!
This is a bad habit if you want to achieve water efficiency in the garden because:
- it increases your water bill;
- it leaches valuable nutrients from the soil (costing you money to replace them);
- it causes loss of oxygen in the soil pore spaces, increasing the chance of root rot and other diseases from suffocation;
- you are eventually wasting a precious resource.
Achieving water efficiency in the garden becomes a sum of good practices. The right amount of watering can save you time and money, as well as water. Also, using rainwater and greywater can help cut your mains consumption.
Remember that most lawns and established trees and shrubs don’t need watering. And last but not least, excessive amounts of water simply drain out of reach of plants’ roots while insufficient water sprinkled on the soil just wets the surface of the soil – and helps the weeds grow!
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