I see a lot of different methods and ideas for watering trees and many times these are over thought. For all the right reasons we want the best for establishing our trees in the first couple of years after planting to give them the very best of starts. However we can over complicate things and end up veering away from the optimum levels of best practice that we are trying to achieve.
Taking it back to basics, trees have evolved to take rainfall as their source of water. Generally this is very slow in terms of output, it rained on and off all of Saturday night until Sunday afternoon last weekend and all that came down was 10mm. But we all agree that the trees look better for it. Rain is slow release water. The rate it is delivered gives vital time for the soil to grip it as gravity takes it down its profile. The canopies of trees have time to channel the flow down and in the process slows it down still further to where its roots system can access it later on. The woodland floor mulch slows it down yet again and keeps it from evaporating as soon as the sun comes out. It also prevents the soil from capping after the relentless pounding of the rain to make the soil more receptive to its flow.
So what do we do?! We pour on buckets of the stuff all at once and walk away thinking good job done! Think of a hanging basket. You water it with a litre of water from a jug and two thirds falls from the basket onto the ground below. That water is now useless to the plants inside the hanging basket. If that litre of water was applied as a block of ice and left to melt over the next few hours, not one drop would fall through the basket as the soil has had the time to grip it. The plants can access this full one litre of water as it is within their root zone. Now, I’m not suggesting you put ice all over the place but it’s a good way to make the point that slow release water is what’s needed. (Having said this I have used ice now and again to water trees and it works very well!!)
So, how to slow water down so that the soil can grip it for the roots to access it? People think water pipes are a great idea, single use plastic protruding out of the soil line after the tree is planted so water can be applied ‘down to the roots’. But the roots are most active and effective on the surface. Using a hosepipe to feed a tube is just going to get the water to sprint through the soil profile with very little being delivered to the right place. Fast release of water can also make a horrible mess of the soil structure below ground. Roots need oxygen and water blended in measure and too much water too fast squeezes out the oxygen from the soil, rendering it useless for tree roots to colonise it.
What about using water retaining gels that the roots can access later? The first thing I think of when I hear about these sort of products is how many water absorbing gels occur in the soil in nature and have trees evolved happily with this type of stuff in place? When push comes to shove and it gets really dry, what’s going to want to hold the water, a root system or a water absorbing gel? I don’t think I’m a technology luddite but you can see the point that veering away from how trees have evolved will probably give rise to negative consequences.
You can’t get away from it, the best way to slow water down is to do your best to mimic rainfall. In my garden I water using a watering can with rose attachment to sprinkle water to the mulch and down through the surface of the soil. A six litre watering can applied every three days is much more effective than lashing huge volumes on at a quicker pace. I can do this as social distancing and self-isolation during this pandemic is giving me a lot of time to use up. So, when time is more pressing, I can mechanise by using a garden sprinkler that I can walk away from and this slows the water rate up even further, more akin to a heavy rain shower so I’m now really getting to where I want to be. About 20 minutes per tree via a garden sprinkler is a great way to water garden trees.
But you can’t set off a garden sprinkler in a town or city street or where you haven’t got a hose point to feed it. This is where Tree Hydration Bags work well. These sit on top of the root system, hugging the stem like a waistcoat. Once filled they expand with 70 litres of water that is then dripped out the base over the next three to five hours. Incidentally they also act as a rabbit protector, weed suppresser and mulch mat. Practically they have to be filled by a bowser or hosepipe to make them preferable to hand watering but they are another great way of slowing up the flow of water for the soil to have time to grip it.
If you mimic the flow rate of rainfall you are on the right side of watering effectively. From the top of the mulch downwards.
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