x Abu Dhabi, UAESunday 23 July 2017

Nick's Garden: How much water does that plant need?

Many variables affect irrigation needs but humans play an important role in plant health and water conservation.

Scientific factors such as climate, root depth, soil quality and irrigation efficiency affect how much water a plant needs, but subjective conditions such as overwatering make a difference, too. istockphoto.com
Scientific factors such as climate, root depth, soil quality and irrigation efficiency affect how much water a plant needs, but subjective conditions such as overwatering make a difference, too. istockphoto.com

From the day I started working in horticulture, when my responsibilities ran from sweeping floors to deadheading petunias, to my more recent work as a landscape architect and now as a gardening columnist, I've faced one question more than any other: "Exactly how much water does this plant need?"

It's a deceptively simple question that rarely produces the definitive answer the eager novice, society hostess or master developer requires. More often than not the result is frustration and anger as the gardener, designer or irrigation engineer responds with a long list of variables and further questions that seem to make the answer even more confusing. In these situations the last thing the questioner wants to hear is, "It all depends…"

Unfortunately, it really does, and there seem to be even more variables in the UAE than elsewhere.

There's a long list of factors that affect irrigation demand, ranging from broader climatic and environmental concerns to effective species selection, plant rooting depth, the water retention capacity of your soil and the efficiency of your irrigation system. The sun, heat, wind and humidity in the UAE are obvious factors affecting irrigation, especially because each of these factors affects the rate of a vital plant process called evapotranspiration.

"ET", as it is sometimes known, is the rate at which a plant loses water through its leaves, stems, flowers and roots (transpiration) as well as the rate at which water evaporates from the soil. On a dry, hot, windy day plants effectively sweat, but when humidity levels are higher the rate of transpiration is reduced as levels of humidity inside and outside the plant come closer together. This is why gardeners who move inland from coastal Dubai often notice that their plants become more thirsty; they are farther from the sea.

It's no surprise that plants that have adapted to harsh climatic conditions require less irrigation than tropical species but it's the latter that continue to dominate gardens and nurseries alike. Many arid species like agave, the native ashuur (calotropis procera) and Texas ranger (leucophyllum frutescens) have leaves that are either thick, waxy or hairy. These strategies help plants reflect the sun, reduce transpiration and capture vital water that appears overnight and each morning as dew.

Other desert plants like the ghaf tree (prosopis cineraria) have deep taproots that can travel many metres below the surface in search of natural groundwater that can sustain them in harsh conditions.

However, even when the most appropriate plants are selected, grouping them together according to their water needs (hydrozoning) and only giving them the water they require are still key factors that need to be taken into account.

If you improve the structure and fertility of your soil by adding compost and other organic material, it will be able to retain more water for longer, particularly if you protect its surface by mulching with chipped bark or cocoa shell. This can help to lower soil temperatures, reduce evaporation rates and should also wick night-time moisture down towards the plant's root zone. Plants grown in such conditions will grow deeper, healthier, more efficient root systems and as long as you irrigate regularly they should become increasingly drought tolerant.

Whatever your garden aesthetic or horticultural beliefs, this is the situation that should be your ultimate irrigation aim: to use less water, less often, more efficiently.

The amount of water a plant needs depends on all these scientific factors but it also depends on some very subjective ones as well. A plant that receives minimal water will grow very slowly and probably end up stunted while a member of the same species that's watered well will flourish. But that doesn't necessarily mean that the latter is more desirable. In my opinion, ancient olives and the mighty ghaf trees that grow out in the desert look all the more stunning because their contorted features display the struggle of their lives.

This is definitely the case with the gnarled trees that grow on the 230,000 hectares of Abu Dhabi's national forest, native ghaf, sidr (zizyphus spina-christi) and mishwak (salvadora persica) of a size and maturity that would be impossible to obtain from any commercial nursery. Grown in the very harshest conditions, these mighty trees survive on only 20 litres of brackish, naturally occurring groundwater a day, whereas their counterparts in our streets and parks receive four or five times as much recycled irrigation water. While the urban trees may be more conventionally beautiful, they're not nearly as awe-inspiring as their desert cousins.

And so the answer to the question of water really does start with a cautious "it depends" because there are so many variables. Ultimately, however, the most important factor is you.

garden@thenational.ae