Garden docs: Dealing with poor soil drainage

L.B. writes: I have a slope behind my house in Rohnert Park and according to what I have read about soils and drainage, there should be good drainage on a slope. But I must be missing something because the drainage is very poor. This is in an area where there has been a good deal of construction with heavy equipment. Can you enlighten me on the subject? One last question: Would using a “liquid amendment” to the soil help the problem?

I hope this explanation helps: Soil with a slope, moderate or steep, while having good surface run-off (often confused with good drainage), may have poor subsurface drainage if the texture is fine, such as soil high in clay, or if underlying soils create a barrier to water movement. Water is always held more tightly in fine soils than in coarse, sandy soils. A fine-textured soil underlain with buried organic matter, sand and even gravel will not drain well. The water will not move through the coarse layer because it is held more tightly in the finer-textured soil above. The best soils for growing plants are uniform in texture throughout the root zone with a good balance of minerals, air and organic matter.

Rohnert Park is known for its clay soil (not necessarily bad). And, given your description of site construction, it most likely resulted in soil compaction and has contributed to your subsoil drainage problem. What to do? Try a subsoil drainage test by digging a hole in the soil about 12 inches deep and at least the diameter of a spade. Pour water in the hole to the rim. Refill the hole a day later and observe how long it takes for all the water to soak in. If the water soaks in within a few minutes, the subsoil drainage may be too good. Such soils may not hold enough water to sustain plant life and can lose valuable nutrients through leaching. If the water takes more than one hour to soak in, the subsoil drainage may be poor. Plants may suffer from oxygen starvation and literally drown.

The best solution is to add organic matter and a good rule of thumb is to incorporate no more than 3 cubic yards of organic matter per 1,000 square feet per year. To put it in simpler terms, it amounts to incorporating amendment 1-1/4 inches deep into the existing soil. Soil improvement is a continuous process and can take many years. Look for organic amendments that are relatively coarse and partially decomposed, such as aged barnyard manure. The type of barnyard manure is not as important as long as it has decomposed (composted) for a year. Fresh manure is too high in ammonia and will injure delicate plant roots.

Liquid amendments, sometimes referred to as “soil conditioners”, are not true amendments and are not substitutes for organic amendments. They temporarily break the surface tension of water around soil particles and allow deeper water penetration, but do not increase the pore space of a soil, nor do they break up clay soil to improve drainage. At best they are only a quick fix.

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Sue Carter asks: Is there a recommended technique to divide day lilies? Mine are overgrown and I find it difficult to divide the clumps before transplanting. There must be an easier way to accomplish this garden task.

Using two spading forks in the center of a clump and prying each fork against the other is one technique, but let’s face it, in reality not many gardeners own two spading forks. Another way is to use a sharp, square-point shovel to quickly chop between the fans. Square-point shovels are invaluable tools, so it is worthwhile to add one to your garden tool inventory.

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Joanie’s garden tip: After several experiments with trying to grow plants under a large eucalyptus tree and all of its accumulated litter, I finally found a lovely perennial that actually thrives and puts out spectacular blooms – foxglove. I thought your readers could plan ahead for warmer weather and try planting foxglove if they have the same problem. Do be careful when planting this perennial if your have small children, because it is poisonous.

Dana Lozano and Gwen Kilchherr are garden consultants. Send your gardening questions to The Garden Doctors, at pdgardendoctor@gmail.com. The Garden Doctors can answer questions only through their column, which appears twice a month in the newspaper and online at pressdemocrat.com.