Sunday, April 6, 2014

Evidence of Poor Drainage

A photo is worth 1000 words:

Poor drainage: water fills a hole in the garden more than 12 hours after a heavy rainfall
More than 12 hours after a heavy rainfall...

Digging a hole and filling it with water to check how quickly the soil drains is considered a standard way to check whether you have soil compact and poor soil drainage.

I didn't intentionally dig this hole to check soil drainage. I dug it in the process or removing an Euonymous alata (a.k.a. Burning Bush), which is listed by the National Park Service as an invasive plant.

I haven't quite gotten around to figuring out what I want to plant there, but whatever it is, it had better be comfortable with periodic flooding.

I already knew the soil drainage was not great in large parts of the backyard. The yard is sloped downward toward the street in the front, so the drainage is better there, but after heavy rains, parts of the back yard can stay soggy for days. When I hired a landscaper to add some plants to the back yard this year, he showed me just how compacted and dense the soil is back there.

I believe that the problem, as in many relatively new developments, has to do with soil compaction caused by heavy machinery. Plus I imagine that the top soil may have been scraped away to sell early in the construction process, with only a thin layer of soil (probably attached to the grass sod) put down when construction was complete. I can tell you there's about a half-inch of black soil attached to the grass in the backyard and then it's heavy, sticky, thick, murderous clay as far down as you can dig.

Thus the subtitle of this blog - Adventures in Gardening on Tennessee Clay!

That being said, the drainage is not equally bad everywhere in the backyard. Perhaps 30 feet away from the hole above, I dug another hole (while removing another burning bush). As you can see below, that hole did not hold water for nearly as long after the same storms had rolled through.

Clay soil with better drainage does not hold puddles more than 12 hours after a storm
Not nearly as much water in this hole, although quite a bit of mulch has washed in

So what's the problem with poor drainage? Roots need water, but they also need oxygen. Yes, roots need to breathe! If soil stays saturated for lengths of time, roots can drown and/or rot.

There are plants that can survive in wet or continuously moist soil, but they are not usually the same plants that are able to survive drought, which is also a regular occurrence in Tennessee.

Similarly, many drought-tolerant plants require well-drained, sandy or rocky low-fertility soils, which are clearly not the prevalent conditions here.

I'm compiling a list of plants that can survive both drought and flooding (plus single-digit winter lows and above-100 summertime highs). For instance, Baptisia australis (False Indigo) reportedly can withstand both severe drought and extended flooding.

I'd love to have suggestions to add to the list!