27 July 2018

Water...or the lack of it

You don’t need to be a meteorologist to deduce that we’re having a dry spell. Richard Barley, Director of Horticulture, shares Kew’s coping strategies.

By Richard Barley

A horticulturist waters plants in the Temperate House

It's hot and dry! 

A short stroll around the Kew site will show you all the symptoms of lack of rain. 

Most years, we receive quite reliable and frequent rain.  Contrary to its reputation, London isn’t an incredibly wet place. Actually its rainfall is delivered fairly evenly across each month of the year.  

2018 has been an exception, and from having a very wet spring (up to early May), the heavenly tap has been firmly turned off and remains in that position. 

Summer rainfall at Kew 

At Kew, the long-term averages tell us that we would normally expect 145mm of rain across May, June and July.  This is quite a bit of valuable and free water which we have missed out on this year.   

Instead, for the month of June, for example, we had only 0.6mm of rain, with a total of 28 rainless days in the month.  

Combined with this miserly lack of moisture has been an unusually high number of sunny days, which makes a change from our usual English summers. 

All of this adds up to an evapotranspiration deficit for plants. This means more water is being lost to evaporation than is available to the plant.

What are we doing about it? 

In short – lots. 

Kew’s collection of plants is one of the most diverse in the world. We have lots of important and rare species growing out in the gardens and within our glasshouses, so it’s essential we protect them in periods of dry weather. 

In times of low rainfall, we have a special system of priority watering that secures water for our most important plants and trees, but cuts back or stops water supply for less important parts of the gardens. 

We tend to prioritise young trees that have not yet established an extensive root system, to maximise their chances of survival. We also prioritise trees from cooler and wetter parts of the world because they may not be very resilient or hardy in hot, dry conditions. 

Lawns are allowed to go brown because they’ll recover.  Routine tasks that use water, such as a power washing of the glasshouses, are stopped to save water. 

Watering in the evening and night-time allows the water to run down into the soil and reach the roots of the plant, without too much excess water lost to evaporation. 

We also apply mulch across the site to reduce water evaporating from the soil. We reduce weed or grass growth around trees, as they compete with the trees for moisture. 

If this weather continues and a hosepipe ban did happen, we would apply for specified exemptions to allow us to ensure that our valuable living collections aren’t put at risk.

Do we store rain water during wetter periods? 

We collect rain water from the roofs of our main glasshouses, which are the Palm House, Princess of Wales Conservatory, and Temperate House. 

This water is used to water plants within the houses, and we also have our own holding tank to collect rainfall.  

Unfortunately, we can’t store enough rain water to meet all our irrigation needs across the site. 

Although we have a lake at Kew, the water from it isn’t suitable for our plants, as it’s too salty (it’s filled from the River Thames at high tides).  

What about the glasshouses? 

When watering the glasshouses, we use rain water when available. This is because the mains water is very high PH, and is high in dissolved salts and calcium. 

When we do use mains water, we have to clean it by running it through a reverse osmosis system. We therefore have these systems installed at all our major conservatories. 

Protecting our plants 

So through a combination of water saving techniques, priority watering and lots of hard work, we’re rising to the challenge of this dry spell to protect our plants.