What Is “Good Drainage”?

When you read a plant description, more often than not the author slips the terms “well-drained soil” or “good drainage” in among the conditions the plant needs to grow well. And true enough, the vast majority of plants, whether small annuals or large trees, do prefer well-drained soil. But what do “well drained” and “good drainage” mean?

A well-drained soil is simply a soil where any surplus water disappears quickly after a good rain or watering. If, on the contrary, you see puddles of water on the ground after a downpour, your soil is not draining well. In poorly drained soils, plant roots can sit in water for long periods and that prevents them from carrying on adequate respiration (they need a regular supply of oxygen), thus slowing their growth or even killing them.

Usually, poorly drained soil is associated with clay, which is made up of very fine particles that tend to clump together so densely and hold water so tightly that excess water remains trapped for a long periods of time. Sometimes the soil was further compacted by heavy machinery used during construction. In other cases, the problem is due to a barrier under the soil (rock, asphalt, caliche soil, etc.) that prevents water from flowing away, or perhaps the water table is barely below the surface. That makes drainage difficult, even if the soil’s texture would be suitable for good drainage under other conditions.

When gardening in containers, the main cause of poor drainage is the use of pots without drainage holes: there should be at least one hole in the pot’s bottom to allow surplus water to drain away.

An Easy Percolation Test

Do you want to know if your garden’s soil well drained? Here is an easy test to carry out:

Dig a hole 1 foot (30 cm) deep and wide and fill it with water. If the water disappears quickly, your soil is well drained. If it takes an hour to drain, the drainage is a little slow, but acceptable for most plants. If there is still water in the bottom 2 hours later, your soil is poorly drained and many of the plants you try to raise there, if indeed they survive at all, will only grow with great difficulty.

If Your Soil Is Poorly Drained

If your soil is poorly drained, the easiest solution is simply to grow plants that tolerate poor drainage! Most willows and poplars tolerate poor drainage, for example, as will plants naturally found in marshes or other soggy soil situations: cattails, aronias, turtleheads (Chelone spp.), joe pye weeds (Eupatorium spp.), ligularias, lobelias, lysimachias, beebalms, shrubby dogwoods, Siberian and blue flag irises, most ferns, etc. You might also want to add a raised gravel path or a wooden walkway to reach the sector, otherwise you’d have to wear boots every time you go there!

But poor drainage can also be easily solved just by installing a raised bed filled with loose soil directly over the poorly draining soil. This’s what most gardeners faced with such a situation do: a simple and fast solution that will give you years of good results!

When the Entire Area Is Poorly Drained

If the entire area is always soaking wet, you can improve the situation by installing (or having someone else install) a proper drainage system (tiles, pipes, French drains, etc.) … but for a drainage system to be able to work, there must be a lower section or drain to which you can direct the accumulated water. Simply putting a layer of gravel in the bottom of a planting hole (a popular but useless solution to poor drainage, although there are still many misguided people who try it!) will give you absolutely no benefit if the water that accumulates in the gravel can’t drain somewhere lower.

An Attractive and Environmentally Solution

Rather than draining water towards the municipality’s probably overloaded sewage system, long the typical solution to poor drainage, builders in new developments are increasingly adding rain gardens to areas where drainage is naturally poor. That way, after a good rainfall, excess water will flow down into a depression that has been planted with plants capable of tolerating occasional flooding (see the suggestions above of plants adapted to poorly drained soils). Thus, the water can slowly infiltrate the soil or evaporate without disturbing anybody. This is both good for the environment (as the water filters through into the soil, plant roots and soil bacteria will filter out any toxins) and for the gardener, as it solves any drainage issues. Plus it attracts birds and other beneficial creatures. Truly a win-win situation!