How often should you change compost in pots?

compost for potted plants

QUESTION: How often should you change compost in pots? I want my potted plants to be healthy, but I don’t want to create too much work for myself. Matthew G

ANSWER: Experts recommend refreshing compost in potted plants on a schedule ranging from every six months (once in the spring and once in the fall) to every year or two.

How frequently the compost in your potted plants will need to be exchanged for a new batch depends on how large the container is. Plants that are growing in relatively large containers can wait until the one- or two-year mark for you to freshen up their potting soil and compost, while plants growing in smaller containers will need their soil and compost changed sooner, closer to every six months. 

Also, some vegetables tend to be heavy feeders, such as tomatoes, peppers, and cucumbers, so they will need their soil and compost updated more frequently. Flowers will not use up the nutrients available to them as quickly, so they can wait longer before the soil and compost needs to be replaced. In fact, if you are growing flowers in containers, as long as they have been growing well and showing signs of good health, you can refresh just a third or so of the soil in the pot instead of replacing all the soil at once. Then top the soil with a layer of compost.

Some gardeners say that you can tell that compost needs to be changed because the compost will visually “slump” in the container, settling toward the bottom. Expended compost will be more densely packed instead of loose and airy. This change happens because as the organic material and nutrients in compost are released and decompose, they settle into the crevices between the grains of compost—spaces that used to hold oxygen when the compost was fresh and new.

The potting soil and compost in containers needs to be revitalized in the case of a few scenarios as well as when the planting medium has expired. Many of the circumstances listed here are times when the planting medium should be updated as a preventive measure, such as to avoid the spread of disease or infestation. Replenish the containers by adding new soil and compost when any of the following situations occur.

  • Plants have stunted growth, are growing slowly, or seem to be in poor health for no discernible reason.
  • Plants in the garden have been infected with a plant disease. You should renew the soil and compost in all your containers where plants are growing even if the diseased plants have been discarded. 
  • One or more of your plants has been victim to an infestation of any garden pest, such as slugs or aphids.

Follow these steps to renew your container garden by adding new soil and compost. Gently remove the plant from its container, using your fingers to delicately spread out the root system. Discard the spent potting soil and compost from the container by pouring it out onto the compost heap or filling in a low spot in your yard.

Fill the pot about three quarters of the way to the top with fresh potting soil, along with a few handfuls of perlite if desired to aerate the soil and make it lighter. Top the soil with a generous layer of fresh compost, then sprinkle a slow-release fertilizer over the compost.

Use potting soil to cover the compost and fertilizer layer, then dig a hole in the middle of the container that will fit your plant’s root system and transfer the plant back into its container. Water the plant immediately after replacing the soil and compost to help the planting medium settle and activate the fertilizer.