The Q&A Archives: Adding Garden Soil to Containers

Question: I like to grow annuals in hanging baskets and window boxes. I use commercial potting soil in each. The soil appears to be composed mostly of peat moss, perlite and, the label says, "wetting agents". Should I be adding some topsoil to this mix? Right now, the containers either seem to stay too wet or dry out too quickly. Do I need drainage stones in the bottom of a 6-inch-deep pastic window box with drainage holes on the bottom?

Answer: Drainage is one of the most important characteristics of a planting container. Most plants dislike wet feet. Roots need water, nutrients and air to keep them happy. The commercial potting soils are usually a combination of sand, peat (or other loose, light organic matter), and perlite. Sand helps the growing medium drain quickly, organic matter helps retain just the right amount of moisture, and perlite adds air space and keeps the mixture light. Wetting agents are added to peat moss, becuase when it dries out it resists moisture.

It's natural for the soil in pots to dry out quickly because the space is confined and plant roots drink up a lot moisture, especially in spring and summer. You should check, and perhaps water, containerized plants every day or two in warm weather.

Organic matter does break down over time, so it's wise to use fresh potting soil each season. Or you can mix your old potting soil with some new potting soil if you think it is depleted, or needs a little more body. But don't add garden soil. It has a tendency to really dry out and be very difficult to re-wet. You don't need gravel or stones in the bottom of your plastic window box. In shallow containers such as these you want to provide the roots with as much soil mass as possible. Just be sure the planter can drain freely. Hope this information sets the record straight and helps you keep your potted plants healthy and happy.

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