Plants are amazing for the way in which they tolerate a wide range of conditions. Their growth depends on light levels, temperature, and moisture availability. The last of these three is the easiest for us to manage.
If there is not enough water, plants can go into survival mode. However, for vegetables, this means smaller harvests.
How much to water is your call, and can be a fun skill to learn. Observe your plants carefully, and aim to give the amount appropriate for each vegetable, the weather, and the harvests you desire.
- A base point is to water less often, though always giving enough water to soak the soil to a depth of, say, 5 cm / 2 in. The result is proportionately less evaporation from the surface.
As a background to some of what I write here, conditions at Homeacres are rarely very dry. Since 1998, the least amount of rainfall that I have recorded in a calendar month is 4 mm / 0.2 in, in June 2018 and also in September 2013.
The annual total averages 950 mm / 37 in of rain, spread almost evenly between all twelve months. Winter is soggy, and in summer we apply water to the plants that benefit from it the most.
Some of my advice does not apply to tropical or arid conditions. However, at the end of the lesson I explain the relative moisture needs of different plants, relevant in any situation.
Sourcing water
Water butts are useful, for as long as their water lasts. At Homeacres, we supplement with mains water through a hand-held hose. It contains some chlorine but not too much, and over the decades I have not observed a problem from this.
Most years I purchase around 100 m3 / 3531 ft3 of mains water, at a cost of £133 / approx. $180. This is for about a third of 1000 m2 / 10764 ft2 of beds, the ones growing salads and moisture-demanding plants.
I consider this an efficient and economic use of water, for sales of over £20,000 / approx. $27,000. Many of these sales are of leaf vegetables, which are full of water.
Applying water
The main choice is between watering by hand or investing in a system of pipes and nozzles, or some type of sprinkler. It depends how you like to garden, and how much time you have available.
By hand
In climates with reasonable amounts of rain, my advice is to hand water. This involves watering the whole soil surface, except when watering new transplants.
To distribute water from butts, use the largest cans you can carry – mine are of 12 l / 2.6 gal capacity, and I hold one in each hand to apply water more quickly. The application rate is therefore rapid, which is fine with no dig because water soaks in so easily. For seedlings and propagation, watering is easier with a smaller can, say of 9 l / 2 gal capacity, and with a finer rose.
We water with the rose of a can facing downwards, to ensure accurate application. This does not hurt leaves and wastes little water, unlike when the rose points upwards.
Hand watering is accurate, but takes longer. The benefits are many:
- You can vary the application per plant, according to what you know and observe.
- You have the option not to water adjoining plants, or spaces that don’t need it.
- You use less water, because it goes only where needed.
- You often notice plants that need some extra care.
Using automatic systems
Although these systems save some time, their distribution of water is often not specific to the needs of each planting. They overwater some plants and underwater others.
Watering through drip lines leaves much of the soil dry, especially near the surface. This decreases microbial activity, and root systems cannot access these often large volumes of dry soil.
This is not the case when you use sprinklers. However, the spatial distribution is hard to match to the area you want to water. Usually they water more or less area than you had intended, and some water is therefore wasted.
Automatic systems also need time to set in place (and later to dismantle). Then, after use, you often need to water a little by hand, where water has not landed. For many of us, they are not essential. However sometimes they are the only option, in which case they can be helpful and serve a purpose.
Moisture needs vary between soil and containers
The no dig method helps soils to hold more moisture, also to drain it sufficiently in wet weather. It therefore works well, with no dig, to give a decent amount of water less often.
This is different from growing in containers, especially plastic ones that do not ‘breathe’. There needs to be air in the rooting zone, as well as water. It’s more likely that we overwater plants in containers compared to the same plants growing in soil.
However, in dry conditions, and when plants are large, containers need more frequent watering compared to soil. In this case, you either need to be around your plants regularly, or to set up a proprietary system.
Most of this lesson is about watering plants growing in soil. Before this, you will often be watering seedlings during the propagation stage. The water requirements of plants at this tender stage of their life are different from when they are older (see information on watering seedlings in Lesson 6 on propagation).
Water needs of seeds, compared to transplants
Based on years of experience, I favour transplanting seedlings to sowing seeds direct – you only need a small space to grow many plants, and watering seedlings in trays requires less water because of their proximity to each other. You save time as well, because you can water tens or hundreds of seedlings in a few seconds, compared to when they are spaced out in a bed.
Once, in May, we sowed pea seeds in a bed after broccoli (see photo below) and only 30% of the seeds germinated as the soil was too dry. Pea seeds are large, and need moisture to swell before they make roots – it was an unusually dry spring and the broccoli had sucked this bed dry.
We knew all of this and watered the drills, but clearly did not wet them sufficiently; we were short of time, and I took a gamble that it would rain. It subsequently rained very little, and we could not see the individual rows so had to spend time and use much water on the whole bed.
There were gaps in the bed from pea seeds not germinating, and I regretted not having had transplants ready. We could have watered them after planting, individually or in rows.
A further benefit of transplants, compared to seeds sown direct, is that they result in quick and even establishment for full use of space.
- Transplants require less water per weight of eventual harvest, compared to direct sowing.
- Give water to transplants to ensure good connection between roots in module compost and the soil / compost surface.
Another saving of water can be from interplanting with module-raised plants. You use the same amount of water for two plantings at the same time.
Watering when soil is very dry
If soil dries out completely, it resists rewetting, thanks to water’s capillary surface being so strong when in contact with dusty, dry matter. It needs time for water to soak in and downwards, bit by bit, hence the flooding from a storm that follows dry, hot weather (as indicated in the greenhouse photo below).
Before planting
In dry conditions, a pre-water of the bed makes it quicker and easier to dib holes for module-raised plants. If the surface is dry to a depth of 5 cm / 2 in or more, your dibbed holes would fill with dusty soil.
However, this does not apply when setting out larger plants at wider spacing because doing this would waste water. It is feasible to use a trowel to make holes in quite dry soil, set plants in deep, and then water well. If it’s very dry, you can fill planting holes with water, then pop in plants once the water has soaked in.
WITH EXTREME DRYNESS
Some preceding vegetables will have drawn out a lot of moisture, such as the broccoli in the example above, broad beans, potatoes, and other large plants.
Another example is when following tomatoes under cover, which you water less during their last month in order to hasten ripening. The soil is very dry after final harvest and clearing the plants, and you need to resoak it to capacity before popping in module-raised plants for winter.
Apply water in stages so that it has time to soak in. Allow a few minutes between each watering and give increasing amounts with each successive application.
The best results come when you water sufficiently to create a connection between the surface moisture and the water table below, before replanting. If soil is still dry underneath a watered surface layer, the zone of dry soil acts as a barrier to any rise of water from below, and roots cannot reach downward to access this moisture.
Before direct sowing
If the surface is dry, it’s best to draw drills and then water along them before sowing, as shown in the photos below, centre and right. You can make the bottom of a drill decently wet where new roots will be growing downwards, which is more important than having moist soil or compost above the seeds.
After sowing, cover with dry soil or compost and don’t water again until new leaves are visible, or even later if they look strong.
Watering in winter, under cover
Some under cover plantings you make in the autumn crop until April, even early May. During midwinter, growth is super slow, and plants only need occasional, light watering. The soil is cool, the air is often humid, moisture levels are not much reduced by new growth or evaporation, and light levels are low. In these conditions, plants are healthier when leaves are not wet too often.
After mid-February, new growth is stronger. Usually it’s good to water weekly from that point, with a little more each time, and then twice weekly from March.
Watering new, no dig beds
Compost holds much water, yet is fibrous and open compared to soil. It has billions of small, air-filled gaps between tiny aggregated lumps. Plant roots easily pass through these, and water can drain through while also being held between the lumps.
However, if there are large pockets of air between big lumps of compost, or of dry material within the compost, plant roots cannot pass through. They need the compost to be firm and even, for stability to grow upright and for anchorage in wind.
Moisture also stays for longer when compost is firm. Therefore, when making new beds with partly dry or very lumpy compost, you need to walk on the compost before watering the beds, to enable it to settle and bind. You can’t ‘compact’ compost by walking on it – you just make it firm, which is good.
Time your watering
It is often recommended that you water in the evening, so that the water lingers for longer and has time to soak in. Also so that the damp leaves can absorb water fully, before it evaporates.
This applies in dry climates and dry conditions. However, in moist climates the opposite works well. Water in the morning when the dew is still on leaves, to add value to that existing moisture. Then the leaves and soil surface can dry before evening – this discourages slugs, and fungal diseases such as mildew.
Watering in sunshine
You can water in bright sunlight because light refraction does not hurt leaves. If it did, there would be many damaged leaves after any storm that is followed by strong sunlight.
This is good news for gardening, because it’s often in sunny weather that plants need water, and it may be the only time we have available to give it to them.
- It’s best to give the water in large droplets rather than as a mist, to reduce evaporation.
Plant responses to lack of water
To discover whether your plants are running low on moisture, look at their leaves on a warm and sunny afternoon. Wilting of a few leaves suggests you need to water, but sometimes it’s not urgent. Leaves of plants like beetroot, and many brassicas, can wilt in dry summer sunshine and then grow again in the cool of a morning dew.
Cucumbers
If ever you notice wilting of several leaves, especially on fast-growing plants, this signals that watering is urgent. Cucumber plants are a dramatic example of this, especially in early summer when the plants do not yet have a well-developed root system but are growing fast.
The result of cucumber plants running short of water at this critical stage is burning of the main growing point. This can result in losing the plant, but fortunately there is a remedy. Plants can be coaxed into regrowing through selecting a sideshoot near the top of the plant to become the main leader.
Moisture needs of different plants
The most efficient time to give a a large amount of water is as plants grow close to their point of harvest, whether that is as fruits, roots or leaves. For fruiting vegetables and potatoes, noticing the first flowers is a signal to water, if conditions are dry.
The information that follows serves as a framework for understanding the moisture needs of common vegetables, relative to the harvests you want from them; they are mostly grouped, either by plant family or type.
Alliums
Leeks are the most moisture-demanding allium, but in dry weather they can survive without putting on much new growth. You may also notice rust spots on older leaves, in which case it’s best to remove them and then give as much water as you can.
Onions and garlic need little extra water until a month before harvest. Give some every five days or so from that point, in dry weather. Stop watering a week before harvest.
Brassicas
All vegetables of this family grow best in moist soil and atmosphere, at any time of year. You want to maintain steady but not excessive growth and, as swelling happens, give extra water to cabbage hearts, cauliflowers, Brussels sprouts and broccoli heads, unless it’s raining a worthwhile amount.
Fortunately, many brassica harvests happen in spring and autumn, when it’s usually damper than in summer. There are harvests in winter too, when no extra water is needed.
- Brussels sprouts may be worth watering in a dry autumn; this also reduces aphid numbers. Adding compost can also reduce whiteflies, because of the extra moisture it can hold.
(There is more information on brassicas in ‘Root Vegetables’ section below.)
Celeriac
Seed to harvest takes seven to eight months, and growth is slow until about half-way. Then finally, in early autumn, the root size increases noticeably.
After watering transplants for the first week, you generally need to give little or no water until late summer. The main time of watering is August and September, if the weather is dry.
Celery
This is probably the most moisture-demanding vegetable. Celery plants never cease to surprise me, in terms of how much water they can suck out of the ground. Give them more than you think is necessary – you should be impressed by the new growth, and will enjoy stalks that are more tender.
Cucurbits
These plants grow exceptionally fast when it’s warm, so it’s often worth giving extra water to increase the harvest.
Courgettes and other summer squash need a lot of water, and you can use watering almost like a tap, to increase and decrease the flow of harvest. The plants are resilient; in dry conditions they may develop powdery mildew on older leaves but this does not affect general health and growth.
A cucurbit exception is winter squash plants. They need water for sure, but have a longer period of swelling their fruits and can cope with dry spells. However, in arid conditions watering is worthwhile.
- You can grow cucurbit plants through holes in black plastic, both to conserve water and to warm the ground at planting time.
- When grown through holes in plastic, you rarely need to water the plants, even through dry weather. The plastic conserves moisture and water trickles into the planting holes.
Legumes
A good time to start watering is once you see the first flowers, if it’s dry weather. Water can also reduce pests, such as blackfly on broad beans, which do most damage to plants stressed by drought.
How much water you need to give depends on plant size. For example, tall peas and climbing beans, which effectively make a hedge 2 m / 6.6 ft high, transpire a lot of moisture when at this stage. They will be flowering and podding too, so water every two days in warm weather.
Root vegetables
These vary in terms of water needs, depending on the season they grow in, the length of growth period, and how you like the roots to taste.
BEETROOT
These tolerate dry conditions, and often show this by wilting their leaves on a sunny afternoon. This makes the roots sweeter, but if you want them to grow bigger it’s fine to water.
CARROTS
They can forage deeply for water, and generally do not require a great deal extra. However in dry weather, giving water will significantly increase the harvest and juiciness of roots, while also reducing sweetness a little.
PARSNIPS
This vegetable can grow slowly over a very long period. At Homeacres we almost never water them, and they are suited tothe often damp conditions and moisture-retentive soil. After dry summers the harvest is less. When the soil is sandy, giving water in early autumn is worthwhile.
POTATOES
It’s rare that potatoes need any watering before the flowering stage, unless your climate is arid. Here, there are seasons when we never water potatoes because it rains sufficiently in early summer. In other years, we water them as soon as flowers appear, and continue for up to three weeks.
- As with cucurbits, you can grow potatoes through holes in black plastic, which minimises the need to water.
RADISH, SWEDE AND TURNIPS
These grow best in moist soil – radish in particular because of its sheer speed of growth – and are more tender when moisture is readily available throughout the growing period.
Salad plants
We want plants to grow many leaves, and leaves are 95% water. The logic is simple: you can increase harvests by frequent watering – every two or three days in dry summer weather.
When you water less, plants do not necessarily suffer and die, but there will be fewer new leaves to pick and they will be less tender.
Tomatoes
These require the most water once they are growing strongly, say two to three weeks after the transplant date in late spring to early summer. From then until late summer, they are both growing and setting a lot of fruit. Water regularly and heavily until early autumn, when you then want the fruits to ripen rather than new growth. At this stage, the plants need very little water.
Other plants
- Aubergines need regular and copious watering in hot weather. Their fruits then swell rapidly.
- Sweet peppers need a fair amount of water, much more than chilli plants.
- Sweetcorn responds well to water, in dry summers and from when you see the first cobs developing.
- Perennial vegetables generally need little water. Their period of growth is long and they have well-developed root systems. Rhubarb is an exception because it grows quickly in moist conditions; plants will not die in dry soil, but will only give small new harvests.
- Herbs in the ground require little watering, with a few exceptions. Mint is thirsty, and is sometimes grown in pots in the ground to prevent the roots from spreading everywhere. In this case regular watering is vital. Basil, once established, repays regular watering with abundant new shoots and leaves.
