Photo Of The Week by Larry Hurley A Tale of Two Yarrows

Most plants you buy will include a phrase something like this: “prefers good drainage.” This means that the soil where the plant is placed should not be subject to standing water after a storm. It either runs off quickly or soaks in quickly. Soil is composed of particles of clay, silt and sand, and some organic matter like dead, decaying roots. It’s also full of worms and microscopic organisms, rocks, aluminum pull tabs and other debris, but in this illustration I’m interested in the clay, silt, and sand.

Clay particles are tiny, silt particles are bigger, and sand particles are the biggest of all. Small particles pack more closely together, while sand particles have a lot of space (pores) between particles. The spaces between particles are filled with air and water. The larger the space, the faster the water drains through it, all other things being equal. So, if you have sandy soil on the Eastern Shore, your garden requires watering more frequently than if you have a clay soil in Gaithersburg.

Plant roots being alive, they require oxygen. When the soil doesn’t drain well and the pores remain filled with water and lack oxygen, plant roots begin to die, and some soil fungi attack the roots, resulting in root rot. So, how do I improve drainage in a clay soil? Incorporating organic matter (compost) helps as over time, decaying organic matter binds clay particles into larger clumps, increasing the air space between particles. You can also add some sand. Don’t walk on, or work, wet clay soil as it squeezes the air out through compression. Planting on a slope or in a raised bed helps, because drainage is driven by gravity and moves from higher areas to lower.

My two pictures. These are two Achillea (Yarrow) planted by me about 6 weeks ago in my Wisconsin garden. Same variety, same size, same grower, planted on the same day. The fuller plant is in the back yard, where the soil has more sand and silt than in the front yard. The bed is also several inches above grade, so, even though a gutter drains very close to this bed, it’s gone shortly after a storm. So, the bed has “good drainage.” This yarrow has new flower spikes and plenty of new growth emerging from the base of the plant.

The sad looking one (which was moved from the original planting area a few days before the photo was taken) had a number of dead shoots, and the roots were nearly gone. This was planted next to my driveway in a soil with more clay, and I didn’t raise the bed because I didn’t want the soil to wash onto the drive. So it’s actually a bit below grade. And I may have been a little too generous with the mulch.

This was not a problem until this year, when we had excessive rain. The soil by the driveway stayed soggy, resulting in the loss of a couple of young peonies, probably this yarrow, and one or more pale coneflowers. I’ve now raised the bed several inches by adding soil, removed some of the mulch, and planted a perennial hibiscus, which should tolerate periodic flooding. Wish me luck!

Achillea