“I’m on my fourth season in a big container at home,” says Mark Highland, founder of Organic Mechanics Soil Co., a Pennsylvania company that produces organic potting mixes.
If large containers remain outside over winter (not a good idea for clay pots), try wrapping them in about 8 inches of straw as insulation to help prevent freezing-and-thawing. Come spring, scoop 6 inches of the growing medium off the top and refill the pot with fresh organic mix; or fluff it up by loosening the disintegrating roots from last season and tipping the mix from one pot to another.
Highland warns that after the first season, a general-purpose fertilizer must be added to the mix, especially if you plan to grow tropicals, annuals, or vegetables. He suggests an organic fertilizer from such companies as Neptune’s Harvest, FoxFarm Soil & Fertilizer Co., or his particular favorite, McGeary Organics. After 4 years or so, the mix will become somewhat dense. When you are done with it in containers, use it on garden beds as mulch, or incorporate it into the garden soil.
If you are new to Organic Mechanics, Highland recommends trying the Container Blend first, as it performs more like the peat-based mixes with which most gardeners are familiar. The Premium Blend has more compost, making it good for window boxes and hanging baskets. Organic, compost-based mixes need watering only half as often as peat mixes, Highland says, so check the moisture level a couple of inches down before watering.