Dahlias Get Drink

Apart from rainfall, the dahlias planted out in spring have been given no water. But because they were planted deeply and because the soil surface has been kept loose for the first 3 to 5 cm, they are still growing. They say that dahlias need lots of feed and water. Well, this may be so. But mine are managing to do with little of each (apart from the bonemeal at planting, they haven't been fed, either).

So how do we know when to water, and how much to give?

I look for the first signs of temporary wilting. Most people rush to get the hosepipe at this stage. I don't - not necessarily. Temporary wilting occurs, usually in hot sun, when the roots cannot supply as much water as the leaves are losing. Once things are cooler, the plants will recover (if they don't recover by next morning, I water immediately).

So temporary wilting is not necessarily caused by shortage of water in the soil (although it could be); it might be because the sun is just too strong, or the leaves are drying too fast.

THE TIME TO WATER

Nevertheless, it is a warning. Once temporary wilting occurs more frequently (i.e. when this is not just a 'once-off'), it is time to water, especially if the plants have lost their bloom (this might be a better sign than wilting).

This is the same dahlia you saw recently. As you can see, the rain has arrived just in time: it is just starting to lose its bloom. It hasn't grown brilliantly. But it has done very well without additional water.


HOW MUCH WATER?

I would give each plant about 4.5 litres (1 gallon), as close to the stem as possible. This should be sufficient to get down to the roots, plus a bit more to to last about a week. This will help cut down on surface water loss (because this water will be held deeper in the soil), save time on watering, and reduce disease and pest problems.

Since every soil is different, it is a good idea to dig down, away from the roots, to see how wet the soil actually is at root depth. Then dig again after a couple of days, once things have settled down, to see how deep the water has gone.

As a rough guide, each square metre of ground will use between 12.5 mm (half-an-inch) and 25 mm (one inch) of water per week during the summer months. One inch (25 mm) is equivalent to applying about 4.5 gallons per square yard.

So we need to apply one gallon per plant at least once, if not twice per week. To conserve water, there is little point watering much beyond the width of the leaf canopy of each plant, which is the extent of the rooting system. This way there is limited wetting of the surface, thus conserving water. And the plants have had a really good drink.