By Elinor Teague
The best time to repot container plants is in early spring, just as new growth begins. Instructions on repotting are basically the same for houseplants and for outdoor potted plants.
Because the soil in container pot subsides or settles over time, many people mistakenly add more soil on top of the existing soil to bring up the soil level. Roots won't grow into the new top soil; roots grow downward. All container plants especially those in smaller 4-, 6- or 8-inch pots should have the potting soil replaced annually even if there are no roots showing through the drain hole and the plant is thriving. Our water here in the Central Valley is highly alkaline with high salts levels. The salts build up quickly in small pots creating whitish crusts on the soil surface and hard ridges of salts around the inner rim of the pot. A thorough flushing of the potting soil will remove some but not all of the salts. Dump the old soil into the green waste bin or the kitchen compost pile and trim off any dead or long winding roots on the plant. Use a wire brush to scrape off the salt residue inside the pot then cover the drain hole with a coffee filter or mesh window screen fabric before replanting. Add a few tablespoons of fresh nutrient-rich compost to the new potting soil if you have some.
Annual soil replacement is not always practical for plants in really large pots. Removing a large plant, emptying the pot and lifting large bags of potting soil to refill the pot requires a couple of strong people with strong backs. Root pruning or root shaving every two or three years will recreate room in the pot for root growth. Use a two-pronged barbecue fork to pry apart the roots then cut back long roots with sharp pruners or shave compacted rootballs with a serrated bread knife or a bonsai root shaving tool until the rootball is one or two inches smaller on all sides and on the bottom.
There are many formulations of potting soils for container plants on the market. Do use sand-based mixes for succulents and cacti, African violet mixes (which often contain peat moss) for African violets, cape primroses and other gesneriad species and orchid barks or moss formulations for orchids. Water retention properties vary considerably among brands of all-purpose potting soils. Experiment (and keep notes) of which brands of potting soil hold water longer.
Many national brands of potting soils add fertilizers to their products but few of those brands list the types and amounts of nutrients on their labels. Some also add water-retaining materials to the mix. It's best to add your own fertilizers keeping mind that, as the water flows out of the drain holes, so do the nutrients. Feed potted plants every two weeks with half the recommended application rate. For flowering plants in small pots, liquid fertilizers including African violet foods with a higher phosphorus content are great. Granular low-number fertilizers are fine for most foliage potted plants. Most Mediterranean herbs need little or no fertilization.