How Often Should I Replace My Window Box Soil?

“The rainfall this year has been incredible and my window boxes are showing it. I usually take off the top of the soil each year and replace it with new soil but this year there is a lot of moss. I would like to know if I should just remove all of the soil and treat the containers or if I can just scrape the moss off and top the boxes off with new soil before planting.” Question from Melissa of McDonough, New York

Answer: All potting soil breaks down over the years, losing aeration, structure, fertility, and drainage capacity.  The dense organic matter of old potting mix just sits at the bottom of pots and slows drainage. Mixes also tend to acidify as they age and break down. Moss grows best on more acid soils that are moist and high in organic matter, which is probably why your boxes have more moss than average.

Simply topping off your window boxes yearly will encourage shallow rooting because plants grow best in newer potting soil, and deep rooting encourages the best growth.

To avoid all of these problems, your potting mix should be replaced every two years or so. If mix from the previous year or two is not well aerated and porous when wet, then it’s time to replenish. (Instead of tossing old mix, I like to incorporate it into my beds as an amendment, so I don’t waste it.) In the process, make sure the holes at the bottom of your boxes are open and draining well. When replacing mix, it also helps to add an all-purpose, slow-release fertilizer.

The best Black Gold potting mixes for window boxes are Black Gold Moisture Supreme Container Mix, which holds moisture well for less watering, and Black Gold Waterhold Cocoblend, which also holds water well and is OMRI Listed for organic gardening.

I hope that these tips help!

Happy gardening,

Jessie Keith

Black Gold Horticulturist


Plants are the lens Jessie views the world through because they’re all-sustaining. (“They feed, clothe, house and heal us. They produce the air we breathe and even make us smell pretty.”) She’s a garden writer and photographer with degrees in both horticulture and plant biology from Purdue and Michigan State Universities. Her degrees were bolstered by internships at Longwood Gardens and the American Horticultural Society. She has since worked for many horticultural institutions and companies and now manages communications for Sun Gro Horticulture, the parent company of Black Gold. Her joy is sharing all things green and lovely with her two daughters.

Leave a Reply

Content Disclaimer:

This site may contain content (including images and articles) as well as advice, opinions and statements presented by third parties. Sun Gro does not review these materials for accuracy or reliability and does not endorse the advice, opinions, or statements that may be contained in them. Sun Gro also does not review the materials to determine if they infringe the copyright or other rights of others. These materials are available only for informational purposes and are presented “as is” without warranty of any kind, express or implied, including without limitation warranties of merchantability, fitness for a particular purpose, and non-infringement. Reliance upon any such opinion, advice, statement or other information is at your own risk. In no event shall Sun Gro Horticulture Distribution, Inc. or any of its affiliates be liable to you for any inaccuracy, error, omission, fact, infringement and the like, resulting from your use of these materials, regardless of cause, or for any damages resulting there from.