Planting Trees in Heavy Clay Soil

Home Grown  by Keith Reed
With fall tree-planting season fast approaching, the time is right to talk about dealing with a common problem the majority of us in Payne County face; heavy clay soils. While some tree species of trees can do fine (some even preferring) these soils, many struggle. Fruit trees such as peaches are one popular example. While it’s not practical to replace the soil, there are some things you can do to improve a trees’ opportunity to thrive.

Heavy soil isn’t all bad-everything else being equal, it is likely going to contain more nutrients and has a greater water-holding capacity than a sandy soil. Drainage (or lack of) is the problem. Roots must have water, but they must also have plenty of air. The roots can’t get to the air if the soil remains water logged for an extended period of time. (Note, there are always exceptions in the plant world. A bald cypress is a great example of a tree that can do just fine growing directly in water).

Roots also need room to grow. The tighter/heavier the soil, the more difficult it becomes for new roots to penetrate the soil. These are the two primary reasons we recommend planting trees slightly above grade as well as digging the hole 2-3 times wider than the root ball, even wider if it is a particularly tough site or important plant since lateral root growth is of greatest importance.

It is not helpful to try to amend the soil in the backfilled hole with sand, peat moss, or some other amendment. In some cases, this can even be detrimental. Think of the planting hole as a clay pot. The roots will happily grow and circle in the amended soil (pot) but will resist putting the effort into expanding beyond the amended area, taking the path of least resistance. This leaves you with a very happy plant for a year or two, but with a severely compromised root system over time.

Complete the planting with a good layer of mulch. As you know, the tighter the soil gets, the harder it gets when dry. A new tree simply cannot out-compete a drying soil for available moisture, especially if the soil begins to crack. Mulch helps insure a consistent level of moisture and a stable temperature, both things the new plant appreciates.

For more information on this or any other horticultural topic, you can contact Keith Reed, the Horticulturist in the Payne County Extension office. Keith can be reached via email at
keith.reed@okstate.edu, phone at 405-747-8320, or in person at the Payne County Extension office, located at 315 W. 6th in Stillwater.

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