Can horticulture go from blue to green water?

#WaterMakesItPossible.  That is so true for the millions of us that have been appreciating our green spaces and growing plants to keep us happy and busy through the recent Covid-19 lockdowns. Getting the watering right is probably the greatest challenge with our efforts either resulting in our plants dying, surviving or thriving.

Although it might seem simple to ‘just add water to the soil’ there is more to it than that, and you can find all the garden watering advice you might need here, on the RHS website. 

The UK horticulture industry is worth £24.2 billion and has a long history of being able to offer us beautiful plants, grown locally but originally sourced from all over the globe, with many of them from places where our climate is similar such as South Africa and the Americas.

But, we all know that our climate is changing, and we will have to adapt our economy, energy sources, lifestyles as well as our gardening practices to be resilient enough to cope with the effects of the changing climate.  Our gardens are often our refuge and escape from these realities and our busy lives, and this has proved itself invaluable to many during the lockdowns.

Water is infinitely recyclable, it always has been, with the cleanest of that recycled water found in our groundwater, rivers and streams, which is often referred to as ‘blue’ water. It is nature itself that cleans it up for us all to use, again and again. 

Plants capture the nitrogen and phosphorous so that it doesn’t contaminate the blue water. Bacteria and fungi in the soil convert carbon into more fungi and bacteria and their exudates, (that’s just another word for bacterial poo), and the soil particles form a very efficient filter capturing the finest particulate matter if the soil is deep and the infiltration of water is slowed down by plant foliage and tree canopy. But nature needs help to do this, and those natural systems such as soil, trees, rivers, streams, peat bogs, gardens and wetlands that we rely on to do the cleaning up for us are under threat from us all using too much water and not recognising the value of the recycling services provided by our wild places. 

Water is perceived to be abundantly available, even though London was one of the top ten cities likely to run out of water in 2018. The vast majority of the UK population are being supplied with a resource that is so essential for our daily life that we don’t have to order it in, and it’s quite literally on tap whenever we want it.  We can normally use as much as we like, the supply is pretty much constant, and we can use it for whatever purpose we choose, whether that’s for drinking, washing, filling a paddling pool or watering our gardens.  But should we?  Is that not how we ended up with the changing climate in the first place?

By thinking that our natural world would somehow keep providing us endless resources and cleaning up the emissions that we continue to emit. What part can gardeners play in helping the natural world do more of the cleaning up and recycling of water for us?

The simplest thing is to reduce the amount of mains water we use in the first place, especially in our gardens.  It can be done but it might take year round planning to achieve.

Gardens are the natural world, our wild places, right on our doorstep, and gardeners can be a positive force in connecting people to nature. Well-informed gardeners understand the water, carbon and nutrient cycles and work with them, especially water. The natural world does not rely on an outside garden mains water tap, an app controlled irrigation system or even a water butt to keep plants alive through droughts or floods, but it also doesn’t try to grow plants that are not suited to the local weather, aspect or soil conditions either. 

In nature, only the best-suited plants survive the native conditions.  I’m not suggesting that we should all stop watering our gardens, if we did, we are likely to lose the benefits of the ‘embedded water’ that has been invested in producing the plant in the first place and lose the many environmental and health benefits that these plants provide. By planning, preparing and choosing plants carefully, understanding their soil and mimicking some of nature’s gardening skills, gardeners might just be able to not only reduce their reliance on ‘blue’ water but convert to ‘green’ water, that’s the water captured in soils from the rain and returned to the atmosphere by evapotranspiration through the plants.

Here’s an example of what I mean.  You could choose to buy this climbing Hydrangea (Hydrangea anomala subsp. petiolaris). It’s a gorgeous looking plant, and will grow in a sunny or shady spot, happily covering a fence or wall in white flowers, even in the shade.  It might even attract some small garden birds for nesting sites once its established and will certainly be home to plenty of insects to feed the birds, so even though it’s not a UK native, we’ve ticked several boxes for adding biodiversity to our garden where there was otherwise just a blank wall.  But it’s only a small plant with shallow roots and will need watering to get it established and it is here that you have choices.  You might consider that you want a sunny wall covered in white flowers and you don’t want to prepare the planting hole in the grey autumn weather, you’ll wait till the spring when it is more pleasant in the spring sunshine, but that means you’ll have more watering to do into the growing season.  Or you could choose to plant the hydrangea in the autumn, in a shady spot that is often quite wet because it drains the water from an adjacent pathway, and you choose to prepare the planting hole with plenty of organic compost and add a thick layer of mulch.

Add a water butt alongside the wall to collect rainwater from an adjacent downpipe and that’s another step towards converting from blue to green water, with more water left in the rivers rather than pumped to our homes and used to water gardens. Planting in the right place, at the right time will not only save you time watering, but the plant will be better established through the autumn and winter months rather than the spring, while the soil is still warm enough to grow roots, and the leaves fall and reduce the need for water. 

The compost will add air to the soil, making space to capture the rain when it comes.  The organic matter will feed the microorganisms around the roots, another box ticked on the biodiversity list, and they in turn will be ready as soon as the warm spring weather arrives to start converting the compost and mulch into soluble nutrients that the plant will use, so no need for chemicals to feed it either. 

The mulch will stop weeds germinating and protect the shallow roots from colder weather should we get any, and by the spring, the plant has already grown roots deeper into the reserves of winter rain still contained in your soil and you haven’t watered it for months. But if there is a dry spell in the spring, it will need water, using water from your new water butt, which the plant will prefer rather than mains water anyway.  So, I hope that explains why a little planning ahead is needed to make the change from blue to green water but there’s more to consider. 

If you forget to water your plants in a dry spell and lose them, you have wasted even more water.  The Hydrangea may have spent 8 months on a production nursery before it reached your garden, watered and grown to perfection, ready for sale.  It might have used 20 litres of water to get to this stage, so the benefit of the ‘virtual’ water has now been lost.  So, don’t stop watering your plants, but take the time to choose the right plants, at the right time, for the right place, to help the whole of the natural world, let’s not forget that plants are really good for the planet.

But that’s just one small plant in one garden, it’s not enough to make a real difference, is it?  Here’s the numbers, the climbing Hydrangea (Hydrangea anomala subsp. petiolaris). is listed in RHS plant finder as available at 84 nurseries across the UK.  Let’s assume they each grow a batch of 48 plants on average, that’s 4032 Hydrangea petiolaris plants in the UK each year, each needing 20 litres of water before, and 30 litres after, it reached your garden.  That’s 202 cubic metres of water.  Switch all of that water from blue to green and we save enough water to supply a family of four people for a whole year.

This is just one plant in RHS Plant Finder, it has around 70 000 entries, where you can find the plants ideally suited to your garden and where to buy them. Switching from blue to green water means just as much gardening, but a lot of mains water saved.

Janet Manning, RHS Water Management Specialist and Cranfield University KTP Associate

Nov 202