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The science of perfect watering

From Sarah's garden to yours

Water is essential for the survival of all plants; it is an instrumental part of photosynthesis, the process where plants convert sunlight into energy. It also forms part of the transport system within plants that moves nutrients from the roots where they have been gathered or from the leaves where they have been created to where they are needed. Last but not least, the pressure from the water in the plant helps it stay rigid and upright, without it, the plant would wilt.

This water, that is so vital for the health and wellbeing of plants, comes from the soil and is absorbed through the roots. It is therefore essential that the soil has a constant supply to meet the needs of the plants growing in it. The key to success is to keep the soil consistently moist to a good depth without yo-yoing between too wet and too dry.

However, there is always the question, how do you know if you are watering enough? There is no easy answer to this question as there are so many variables. The water availability is not just dependent on the type of soil you have or the season, but your individual microclimate and the type of plants you are growing.

Man watering

If you have a sandy soil, water will drain freely and easily so you will need to water more frequently. A clay soil holds on to water tightly so it doesn’t lose it as quickly, meaning it needs watering less frequently. There are many home tests you can do to determine your soil type from adding water to a handful of soil and trying to make a ring from it. If you can’t get it to hold its shape then it is quite sandy and if you can easily make a complete circle your soil has a lot of clay.

You can also work out what kind of soil you have by adding a trowel full into a jar of water and giving it a shake and then leave overnight to settle. The next day you can work out the proportions of each layer which should be clearly defined. The coarse sand will be at the bottom and the finer clay will be at the top.

While it is good to have free draining soil, you don’t want it to drain away too quickly, or it can leach out the desirable nutrients. Water that takes too long to drain away can be harmful as perpetually soggy soil can cause root rot in plants over the long term. An effective way to find out what kind of drainage you have is to dig a hole about a spades depth and fill it with water and then see how long it takes to soak into the surrounding soil. A good drainage rate is when the water disappears between 15 minutes and an hour. This means the soil can hold enough moisture to keep plants happy on a hot sunny day, but not be so wet that it can cause harm to the roots.

If your soil drains too fast or too slow the answer is to add compost or organic material to the soil. In sandy soils it will improve moisture retention, and in clay soils it will break up the bonds that lock the moisture in too tightly. The added advantage is it will feed the micro communities who in turn will improve the structure of the soil and release nutrients the plants can use.

Once you have the right soil that is receptive to the right amount of water, you still need to make sure you deliver the right amount of water when the rain isn’t taking care of things; time of day can make a difference. The best time to water is first thing in the morning when the soil is most receptive to being watered with the least amount of evaporation. Late afternoon or early evening is another suitable time to water, however if you leave it too late you can create a humid environment around your plants that can invite fungal problems. That being said, taking into account all the demands of modern life, the best time to water a garden is when you have the time to do it.


An alternative to hand watering your garden is to install an irrigation system or a sprinkler and attach a water computer to your tap. You can program the water computer to water your garden at the ideal time, in a way that directs the water onto the garden where it is needed via drippers and sprayers in the irrigation system, or that covers a larger patch if using a sprinkler. This way the garden can be taken care of before you even get out of bed in the morning, giving your plants an ideal start to the day.

When watering, it is better to give a deep watering every couple of days rather than a short sprinkle every day. If your soil is good at retaining moisture then you may only need to water once a week, but for more free draining soils, you would be better off watering every other day. Make sure you water deeply as a shallow watering encourages roots to develop just below the surface of the soil, where they are at risk from the drying effects of the hot midsummer sun. A soil wetter can help to ensure the moisture penetrates deep into the soil.

The kind of plants you are growing also determines how much watering you need to do. Lavender likes things a lot dryer than a thirstier canna lily. It is important to find out what your plants like, so you don’t kill them with kindness. Some plants will need much more than others, for example, in hot weather, a tree will need at least 38 litres of water for every 2.5cm of its trunk diameter every week.

This still doesn’t answer the question – how much water is enough? There is a general theory that states that a garden needs between 2.5–5 cm of water each week - but what does that actually look like? The best way to find out is with trial and error. After watering, dig down beside your plant, but far enough away so you don’t damage the roots. If the soil is moist down to 10 cm or more then you are doing well. You can also get a soil meter to show you how dry the soil is getting or put a rain gauge in where you are watering to see how much is being delivered to your plants.

Your plants will soon tell you if you are watering too much or not enough as they will wilt easily, or the leaves will turn yellow and drop off. These are signs for over and under watering. However, with more established trees and plants the damage may not be evident for several months.

A good thick mulch can help lock moisture into the soil to reduce the need for frequent watering and has the added benefit of preventing weeds.

So, at the end of the day, knowing how much to water your garden comes from knowing your garden. Once you work out what it takes to meet its need, you will have a healthy, flourishing garden to enjoy.