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House Plants Need Repotting

Just mulching won’t give them what they need to overwinter

The editor of Bay Weekly recently asked if she could simply add potting soil to raise the level of rooting medium in her houseplants or if she had to repot. I advised her to repot.
    The rooting medium in house plants shrinks with time. As the organic matter in the potting medium decomposes due to high soil temperatures and frequent irrigations, the volume of rooting medium shrinks. If there are drainage holes in the container, most likely some of the finer particles in the rooting medium have been washed away. As the root ball of the plant grows smaller, it dries out more rapidly so the plant needs to be irrigated more frequently. Eventually the plant becomes root-bound and stops growing.
    Simply raising the volume of rooting medium by mulching the plant with new potting soil does not provide the roots with any relief. The roots do not have access to that surface layer of potting soil because the roots of plants grow sideways and down. The top of the plants grows up. A classic science project is to try to grow plants upside down. The top of the plant will always turn towards the sky, while the roots will turn and grow downward.
    When repotting, always shake away as much old potting soil from around the roots as possible. If the roots are crowded and circling the inside wall of the container, pull them outward or cut the roots to promote branching and formation of new roots.
    Take the soil that you have removed and blend it with equal parts new potting soil and add at least one-third to one-half by volume of compost. Place sufficient soil in the bottom of the pot to raise the planting depth of the surface roots to within one inch of the top edge of the pot. Work your improved potting soil down around the roots.
    Bounce the bottom of the pot on a hard surface several times as you press the soil down along the stem and between the roots.
    After the soil is adequately worked around the stem and roots, allow at least one-half to three-quarters inch of space for watering.
    Water thoroughly so that excess water can be seen dripping from the bottom of the pot.

Houseplants Need Lots of Sun and Little Fertilizer over Winter

    I just read a column from Behnke’s that plants indoors for the winter should get as much light as possible but no fertilizer.
    I have always fertilized indoor plants. I generally use a weak mixture of MiracleGro once a month in winter, twice in summer. They seem happy.
    I just picked up some Osmocote that says it lasts for four months. Should I use it?
    –Lynn Whitall; via email

    Houseplants need all of the light they can absorb during winter months, so put them in front of windows.
    Most of the Osmocote fertilizers release nutrients for three months or more, and Osmocote 18-6-12 releases for eight months. Keep doing what you are doing in winter and save your Osmocote until next spring. At that point, drill the Osmocote into the potting soil by poking a hole with your finger about an inch deep. In the hole, add one teaspoon per six inches in diameter.

Ask Dr. Gouin your questions at [email protected]. All questions will appear in Bay Weekly. Please include your name and address.