The Herald - The Herald Magazine
Tap tip Keep on watering plants through warm spell
WATERING is a gardener’s priority just now. I’m the last to complain about the best summer we’ve had for years, but without a regular supply of life-giving water our plants certainly will.
To grow well, plants need air, water and nutrients. There’s a balance between very fine and larger pores in soil, acting almost like a sponge. The small pores contain moisture and the larger ones lay on the air.
As water run out, an annual plant wilts and dies because roots can’t spread quickly enough to absorb nearby water. But perennials usually develop a relationship with mycorrhizal fungi which collect liquid from a larger area.
You’ll have had fewer problems if, in spring, you worked to improve the ground with compost and good organic mulches. These keep the soil surface moist and prevent evaporation. But remember: dry soil remains dry when mulched.
Well structured moist soil easily absorbs rainwater, but one or two showers hardly affect sandy soil because they only wet the surface.
Very roughly, a millimetre of rain represents one litre per square metre. But it’s estimated that many plants need 2-3cm of rain every week.
This equals 25-30 litres per square metre. In this weather, field crops need around 10 tons of water per acre every day. So it’s hardly surprising you see farmers watering their tatties even when it’s raining.
All this also works for containers. Well-structured home-made compost retains moisture for much longer than commercial compost where water rushes through alarmingly quickly.
A plant saucer helps, but you may simply have to wield the watering can more often.
Soil quality also affects nutrient levels. In 2008, a study compared the effect of drying soil on unimproved and grassland treated with artificial fertiliser. Nitrogen levels didn’t change in the former, but significant amounts of soluble nitrogen were leached away from the latter.
THE scientists found the larger community of micro-organisms, especially fungi, in unaffected ground explained this. So, depending on your soil, you may need to keep nutrient levels topped up in pots and open ground while plants are developing.
This especially applies to annual fruit and veg.
And as I write, forecasters are predicting this wonderful spell of weather will continue, so keep on watering. It’s frankly astonishing
how much water plants need. And although you can help reduce evaporation, you can’t stop plants cooling themselves by transpiring.
The larger the leaf, the more a plant transpires, so a courgette or cucumber is a water junkie. Narrow-leafed rosemary and dianthus have modest requirements, so you should use the head when watering and only give to needy cases.
So what’s the best way of watering? When running a demonstration garden, we tried lots of different systems for containers; they helped but didn’t perform as well as manufacturers claimed.
I’m convinced hand watering is best. You water according to the variety and a simple finger probe shows when it’s needed. You enjoy the plants by getting up close, tie in or deadhead, and spot problems at an early stage. A couple of good soaks a week works for many pots.
In the open ground a leaky hose works well, even though plants at the bottom of a slope get more water than those at the top.
But if you have pliable irrigation tubing, you’ll get a more even distribution by weaving it through a bed in S-shapes.
Always use a pressure reducer with mains water. My gentle gravity-fed spring water needs no such device.
Otherwise a spray gun on the hose is the answer. Salvia nemorosa Caradonna. A perennial salvia with rich purple flowers in spikes that last for much of the summer. Its narrow, grey-green leaves ensure that, once established, it is drought-tolerant.