Got Clay Soil? Gravel to the Rescue

September is often touted as the month when you can plant hardy perennials, shrubs and trees, giving their root systems a good start while the soil is warm. Even if you’re already thinking about restocking the greenhouse, you can still prepare your outdoor garden by adding amendments, and loosening the ground to get a jump-start on next spring. However, if the thought of turning over one more clod-laden sticky mass of gardening dirt—I won’t dignify heavy clay with the name soil—makes you want to throw in the trowel, consider the experimental work of Maurice Horn, owner of Joy Creek Nursery (www.joycreek.com) in Scappoose, Oregon.

Maurice amends his flower beds with gravel. Last spring, I toured Joy Creek’s display gardens, while he preached his gospel of gravel. Burgeoning flowers flourished all around me with low or even no additional summer water. He claims the gravel lightens the soil, improves drainage, reduces water consumption and moderates the soil temperatures. But not just any old gravel. Clean crushed gravel. Around the country this gravel goes by many different names.

Here in the Northwest, what Maurice uses is called quarter-ten, without fines or minus, which means the washed gravel is free of small particles. He says it must be sharp-edged, not rounded pea gravel. The jagged texture allows air pockets in the clay, making it more porous so plants root better and take up nutrients more easily. Best of all, the gravel stays where it’s put, not breaking down like organic amendments. I joined the choir. I dug up a section of my full-sun perennial bed, added the gravel, planted and mulched. While it will take several seasons to know the efficacy, so far I’m happy with my new tilth.

Here’s how I followed Maurice’s recipe. 

• Loosen clay with a fork to the depth of the tines.
• Add on top two inches of sharp clean gravel.
• On top of gravel, add two inches of compost, or other organic amendment.
• Dig in to approximately eight inches with a spade. Mix well—the gravel will look like chocolate chips in cookie dough.
• For each planting hole, sprinkle in a handful of organic food with numbers like 4-6-5, or 4-4-8 to give plants a beginning boost.
• Spread on two inches of your favorite mulch. Keep mulch away from growing stems to avoid problems with rot.