Not your usual gardening tips - Save Water to Keep the UK Beautiful

By Janet Manning, Water Management Specialist RHS/Cranfield University. Janet’s work is funded by UKRI, through Innovate UK,  part of the Knowledge Transfer Partnership (KTP) programme.

Saving water in the garden – getting to the root of the problem

Garden watering seems straightforward, but getting it right is key to growing plants well. Enabling seeds to germinate and plants to produce flowers and fruits for us to enjoy, all requires the right amount of water at the right time and in the right place in the garden. Getting it right can be hugely rewarding in floral displays and yields of fruit and veg to eat. Despite the lockdown, we will be seeing plenty of inspiration to keep us all growing during next week’s Virtual Chelsea Flower Show and I hope this will help everyone to continue to gain all the benefits that gardens bring without wasting a drop. I spent last summer running an irrigation trial at Wisley for the Royal Horticultural Society. One of the main findings was that most water was wasted in hanging baskets when the soil moisture in the compost was either very low or very high. Our findings are helping to shape new watering advice.

The trick is keeping the water ‘in’ the pot

How do we know that we are using enough or too much water for our plants? And, most importantly, how do we know if the water has reached the roots, where it needs to be. Very often we don’t, but because water is perceived to be cheap and readily available, we tend to add a little extra just to be sure. Plants show more negative responses from under-watering than overwatering. Containers and
hanging baskets do need more frequent watering than most other areas in the garden and the usual advice is to ‘water until it runs out the bottom of the pot’. It seems obvious, but the water is only useful to plants if the water is in the pot, because that’s where the roots are. So if it doesn’t reach the root tips or if it keeps travelling, past the root tips and out the bottom of the pot (taking valuable nutrients with it) then the water is being wasted. There are two reasons why this advice doesn’t always mean that you are watering your containers well. Firstly, if the compost is very dry it doesn’t absorb the water, just like trying to mop up a spill on the kitchen floor with a really dry mop, it just
doesn’t work. Once the mop is damp, but not saturated, it absorbs the water. It’s the same for compost. So watering a really dry pot until it runs out of the bottom of the pot, probably means the water has simply run down the side of the container (where the compost has shrunk away from the sides as it dried) and straight out of the drainage holes at the bottom, and bypassed most of the compost. It won’t have absorbed as much water as the plants need and most of it has been wasted, and you’ll be left wondering why it all went wrong and your plants have wilted when you followed the advice precisely.

How much is too much?

Secondly, the same thing happens if you water your compost when it is already very wet. It just can’t retain more water and excess then runs out the bottom of the pot, usually taking valuable nutrients with it. One easy solution is to place a saucer under the pot (or a bucket under a hanging basket) and catch the drained water to re-use it. If the pot was dry, you will see how quickly the water will be soaked back up into the pot from below, and if you have been overwatering, it will sit in the saucer for days. If you are keeping your containers well watered, you are missing out on a natural phenomenon that plants use to optimise their water use. When the roots become partially dried, the plant naturally produces a substance called abscisic acid, which tells the leaves to partially close its stomata, to reduce the water loss. In other words, you can train your plants to use less water by giving them slightly less water. Or to put it another way, if the glass is half-full, they drink more slowly.

How to keep the glass half-full

So, rather than telling you all the usual tips for saving water in the garden, (which all still apply and you can find them on the RHS and Waterwise websites) I’d like you to consider a different way of knowing how much water is enough for your plants, when you can’t see what’s happening in the soil.

  1. When you first plant up the container, make sure the compost is well watered so that the
    compost settles in around the roots
  2. Note the volume of the container, a 30 cm hanging basket is typically 5 litres
  3. The next time you water add just 10% of the volume of the container, so for a hanging
    basket that’s 500 ml, about a pint, add it very slowly, aiming to keep it all in the basket
  4. Gently lift or nudge the container to check how heavy it is, if you think it still feels light,
    give it some more water
  5. If no water has yet drained out of the bottom of the pot, add a little more but aim to
    never let the water run out of the container
  6. Repeat daily. For warm, dry windy days, adding 10% of the volume of the pot won’t be
    enough and the plants will draw on the reserves of moisture but on cloudier cooler days, it
    will be too much and those reserves will be replenished.
  7. The container is likely to never reach its full capacity, meaning that the partially dry roots
    will produce the abscisic acid that reduce the water use of the plants.

Looking back at my results from last summer, this is how some of the best performing baskets were watered but received even less, just 7% of their volume each day. Some days they could have done with some extra (but they didn’t get it, as that was how the trial was planned) and when we had 18mm of rain over 2 days in August, they were completely topped up again. If you feel this is a small, insignificant contribution to make in saving water consider this. If the 25 million gardens in the UK were able to each save just one 10 litre watering can of water during this summer, then enough water would have been saved to supply all the people in Bristol, Leeds and Sheffield for a whole day. Gardeners really can make a difference.

One of these baskets received 30% less water but grew only 5% less flowers, saving more than a watering can (13 litres water) over a 9-week growing period for one hanging basket. Can you tell which one?