The clear blue skies of the last week have given way to changeable weather with regular short showers, some heavy some not. This shower yesterday morning was so light that next to no water reached the roots of the plants. You can see the dry shadow of the hostas clearly outlined on the boards of the terrace.
Because of the number of climbing plants rooted in them, the plant boxes on the terrace support a large area of leaf from a small surface area of earth (or potting compost). In the growing season, it takes a heavy shower or two to provide a day’s water supply for the plants. It’s counterintuitive to go out with a watering can when it’s been drizzling all day, but these plants have captive roots – they can’t seek out their own water.
How much water do the plants actually need? Ideally at this time of year I’d like them all to be growing unchecked, to develop their full potential. The two small plant boxes each have a surface area of roughly 0.3 m2, well covered with foliage. Each also supports climbing plants covering both sides of 1.2 m2 of wirework, so the total area of plants supported by the roots in each box is roughly 2.7 m2. The larger boxes have 0.6 m2 of surface area, but the climbers growing in them have their backs to a roof, so the ‘functional’ leafy surface area of each is maybe 3.4m2.
A general rule of thumb is that temperate garden plants use roughly one inch or 2.5cm of water a week in summer. One litre of water per square metre supplies 1mm of water, so each square metre of plants needs 25 litres of water per week, or two and a half watering cans full. So, the smaller boxes need nearly seven cans full a week each while the larger boxes could make do with eight and a half each. Instinctively I’ve been giving the smaller boxes about a can a day each in warm weather and the larger boxes twice that. No wonder the ferns have been growing so well!