How to Improve Soil Drainage Without Digging Up Your Entire Garden

To determine the severity of drainage issues, it is suggested to conduct a percolation test. You can do this by digging a hole approximately 12 inches deep and 6 inches wide, fill it with water, wait for it to drain completely, fill it with water again, and then time how long it takes to empty.

This test can give you a rough idea of the water infiltration rate. For example, well-draining sandy loam soil usually has a percolation rate of 0.5 to 1.0 inches per hour, while clay soil can be as low as 0.05 inches per hour (UC ANR). If your soil falls into the latter category, there is a severe structural issue requiring attention.

Why aeration beats tilling every time

The knee-jerk response to drainage issues is to tear the whole thing up and 'fix' what's wrong. This can indeed break the cycle of soil degradation because you are breaking the symbiotic mycorrhizal fungus networks your plants have been encouraging for years. Additionally, once the soil has been settled again, things can actually be more compacted than they were previously.

However, core aerating is a better place to start. Unlike spike aeration that just pushes some soil to the side and compacts the rest of it further core aeration literally pulls plugs of dirt out of the earth. Those plugs aren't merely proof that the holes were dug, they create open channels immediately that will bypass the compacted layer, enabling water to drain before it turns into surface runoff. One session of core aerating can even produce a result that is observable within a few weeks for a lawn or bed that hasn't been touched in years.

You will see the difference in the percolation rate after core aeration, and it most certainly isn't the placebo effect. You aren't changing the level of clay - you are simply creating macropores that invite the water to pass into the profile instead of inviting it to sit on its surface.

Fill those holes to make the improvement permanent

Just aerating the lawn won't work for long if you neglect to do the follow-up. The holes fill back in within a season and you're back to square one, and the whole exercise was pretty much a waste of time.

The holes are always supposed to be filled with some kind of coarser material in standard professional practice and you typically see them do this with bunkers on golf courses the day after they've been slashed. Filling freshly aerated holes with top dressing sand holds those channels open as the surrounding soil settles and you won't mold over those holes 'til you're sick of shoveling. Plus, it permanently alters the soil texture in the root zone. Not by replacing the clay, but by introducing enough coarse particle material that H2O has a path to follow. A stiffer brush or drag mat material (not just any mat) will help work the sand downward rather than leaving it as a layer on the surface.

One warning here - A little bit goes a long way. You want a thin, even layer that fills the holes without over-burying the grass if you do get too much in there at once after a few applications you may find you're left with the dreaded hardpan layer as the interface between topsoil and sand.

Vertical mulching for trees and shrubs

Vertical mulching is a useful technique when dealing with drainage issues in established trees or shrubs. Meaning, in such cases, you must work around the existing root system.

This process creates a hole every 30 cm around the tree to a depth of 45-60 cm. These holes are then filled with an inorganic material such as gravel. This allows water to penetrate the soil more quickly, helps channel oxygen and water directly to the roots, and gives roots more room to grow.

Build toward a no-dig system over time

The procedures mentioned earlier will take care of the current issue. However, the ultimate objective is to have soil that naturally manages water.

Did you know that when earthworms burrow, they actually improve drainage better than most artificial methods? This is because, while moving through soil, a single earthworm creates a series of channels underground, which are the ideal underground structure for enhancing water penetration. To encourage the presence of more earthworms, you should reduce your tilling, add mulch over any exposed soil, and allow cover crops to decompose in the soil instead of removing them.

It's also important to manage thatch. A thick layer of dead plant material between the soil and grass makes it harder for water to penetrate, before passing through the root area. You can control this by either dethatching or using scarificationannually, these two methods help in ensuring the layer of thatch is kept minimal so that water can easily penetrate the soil.

The no-dig approach has its reason. Repeatedly disturbing the soil structure actually hinders the desired drainage, it doesn't facilitate it. The organic material that is added on top such as mulch, compost, and cover crops naturally makes its way down through the soil that slowly but surely helps the soil by creating more space for natural drainage.

So aerate to relieve the immediate pressure, top-dress to buffer to your soil's strengths, and let nature do the digging. In three years, you'll have drainage that puts total renovation to shame.