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Some houseplants have to be repotted every six months, while others can stay put for two or three years. Frequency of repotting also depends on container size, quality of care, productivity of the rooting medium and frequency of nutrient applications.
Annuals — such as grape ivy, begonias and marigolds — have very vigorous habits of growth and should be repotted at least twice yearly. Foliage plants such as ficus, schefflera and crotons tend to grow slowly and can be left alone for a year or two, depending on the age of the plant and container size.
Step 1: If root-bound, repot
As you move houseplants in for the winter, check first whether they are root-bound. To see the root ball, knock the plant out of its container. Holding the still-potted plant with your fingers on each side of the stem, turn it upside down. Rap the top edge of the container sharply on the edge of a solid table or bench, and the root ball will dislodge. If it is covered with a solid mat of roots, the plant is root-bound.
To stimulate root-bound plants to produce new roots, take a sharp knife and make four or five cuts through the root mat from the top to the bottom of the root ball. Using your fingers, loosen as many roots as possible and shake out old rooting medium from the center of the ball.
Unless the roots of a root-bound plant are disturbed during repotting, the plant will stay root-bound despite having fresh rooting medium.
If possible, transplant into a larger diameter container. If using a larger container is not feasible, apply the bonsai root-pruning practice, cutting out one-third of the root mass to allow new roots room to grow.
Step 2: Use active potting soil
Repot into freshly blended potting medium. Try this recipe. Mix equal parts by volume garden soil (less for plastic or ceramic pots), compost from your garden or commercial compost and perlite. Place in a microwaveable container and microwave at full power for 15 minutes for each gallon of potting soil. Cool before using. Store the unused rooting medium in a plastic bag so that it will remain moist.
Or improve commercial media by adding one-third by volume compost such as LeafGro. Peat moss-blended media shrink over time; avoid them.
If you have old potting medium, whether homemade or purchased, make sure it is biologically active. Old potting soil that has been allowed to dry out and remain bone dry for months is biologically dead. To make old dried-out potting medium usable, moisten and blend it with either fresh compost or new potting medium.
Add all the old rooting medium to your compost pile.
Place some fresh potting medium in the bottom of the container. Replace the plant, adding and tapping down more medium as you go. Using your thumbs, press the rooting medium firmly into the center of the root ball and between the root ball and the walls of the container.
Leave a half-inch free space between the top of the root ball and the top edge of the container for proper watering. Finally, bounce the bottom of the container sharply on a hard surface so that the loose potting medium fills in the voids.
Step 3: Water generously
As soon as you finish potting, flood the surface with water several times until you see excess water flow from the bottom of the container. Flooding the surface of the rooting medium washes it into cavities. After the water drains, fill with additional medium. Allow to drain thoroughly before bringing inside.