How deep does watering actually need to go, and how do I check?

Most watering is wasted, and the person doing it has no idea, because there is no feedback. Two minutes with a spade converts watering from a hopeful ritual into something you can actually verify.

What you'll need

• A spade
• A long screwdriver or a soil probe
• A bucket or watering can with a known capacity

Step by step

1. Water as you normally would.Do not improve your technique for the test. The point is to find out what your usual watering actually achieves, so do exactly what you do on a normal evening.
2. Wait about an hour.Water needs time to move down through the soil. Checking immediately shows you a surface pool; checking an hour later shows you where the water actually went.
3. Cut a slice with a spade beside the plant.Push the spade in vertically a hand's width from the stem, lean the handle forward to open a clean face, and look at it. Do not lever a plant out — you are opening a window, not digging it up.
4. Read the colour change.Wet soil is visibly darker than dry soil. The line where the colour changes is the depth your water reached. On many gardens after a normal watering it is shockingly shallow.
5. Compare it against where the roots are.Look at the cut face for roots. If the wetted band stops above the bulk of the root system, the plant received nothing useful. That is the whole diagnosis.
6. Adjust the volume, not the frequency.If the water did not get deep enough, apply more in one go rather than more often. A single large application penetrates; the same total split into daily doses never gets past the surface.
7. Re-check after you change the routine.Cut another slice a week later with the new regime. This is the only way to know whether the change worked, and it converts a rule of thumb into a measured fact about your soil.
8. Use the screwdriver as the quick daily version.A long screwdriver pushed into the ground slides easily through moist soil and stops where the soil is dry. It is rougher than a spade slice but takes five seconds and is good enough for a routine check.

Tips

• Different soils behave completely differently. Sand takes water straight down and holds little; clay resists it at the surface and then holds it for a long time. Your check tells you about your soil, which no general advice can.
• Bone-dry soil repels water. If it beads and runs off, wet it lightly first, wait ten minutes, and then apply the main volume — or make a shallow basin around the plant so the water cannot escape sideways.
• Watering slowly beats watering fast. A hose trickling at the base for several minutes gets far more water into the ground than the same volume blasted on in thirty seconds and running off.
• Mulch after watering, not before. Mulch on dry ground keeps rain out as well as keeping moisture in.
• For established trees and shrubs, water at the edge of the canopy rather than at the trunk. The absorbing roots are out there, not next to the stem.

Common mistakes

• Watering a little every evening — It never penetrates, almost all of it evaporates the next day, and it draws roots up into the surface layer where they are entirely dependent on you. A plant watered this way is in far more trouble if you go away than one watered deeply once a week.
• Judging by the surface looking wet — The surface looks the same after five litres and after fifty. The surface is the one part of the profile that tells you nothing.
• Using a sprinkler on borders and assuming it did the job — Sprinklers deliver slowly over a wide area and most of it evaporates or is intercepted by foliage before it reaches the soil. Check with a spade before trusting one.

Questions people ask