“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!
Black Gold Horticulturist