Why Claret Ash Trees Fail to Color in Clay Soil—and How to Fix It

Question: My claret ash trees along the driveway won't develop autumn color, but a damaged tree potted in potting mix colors beautifully. Our soil is clay with an iron pan. What spring fertilizer will improve fall color next year? (Manawatu, North Island, New Zealand)

Answer

The potted tree's vibrant color hints at the problem: nutrition and drainage. Clay soils often lock up nutrients and restrict root growth, while the loose potting mix allows optimal uptake.

Fertilizer Strategy

Apply a balanced 10-10-10 fertilizer at 1 lb per inch of trunk diameter, scattered evenly under the canopy and watered in well. Do this now and again in late summer. The phosphorus (middle number) supports root development, while potassium (third number) enhances flower and fruit color—and by extension, foliar pigment.

Soil Improvement

• Mulch 3 inches deep with organic material (pine straw works well) to conserve moisture and reduce grass competition.
• Avoid fertilizer spikes—they concentrate salts and can burn roots.
• If possible, aerate around the drip line to improve oxygen exchange in the clay.

Healthier roots mean better nutrient uptake, which translates to the brilliant autumn display you're after.