Volume 13, Issue 45 ~ November 10 - November 16, 2005
The Bay Gardener

By Dr. Frank Gouin

Repot Your Houseplants

The dirt on potting soil and soil-less plant media

Fall is the time to repot houseplants. During the short daylight hours of late fall, winter and early spring, most houseplants don’t produce much top growth. This rule is especially true of plants that live outdoors during the summer.

For a plant to grow in a container, there must always be room for new roots. Plants are root-bound when their roots fill the pot. Root-bound plants generally stop producing top growth, and they often start blooming profusely. If the roots are left undisturbed, the plants often develop mysterious symptoms. If you ignore the symptoms, the plants deteriorate.

Repotting does not necessarily mean putting plants into larger containers. Most house plants can be repotted by simply removing the root ball from the container, shaking the root ball to loosen the roots, cutting out some roots and cutting some roots in half to make room for more rooting medium. The freshened plant can be returned to the same container.

The healthiest plants grow in matched pots and soils.

Most commercial potting materials contain mostly peat moss, perlite or vermiculite and milled pine bark. These soil-less rooting media should not be called potting soil. They are generally amended with commercial fertilizers sufficient to support plant growth for six to eight weeks. Unless you fertilize these plants after two months of growth, they often show nutrient-deficiency symptoms such as yellowing or dropping bottom leaves.

Amending commercial medium by one-third volume of compost, such as LeafGro, improves them and reduces your need to fertilize.

You can achieve better results by making your own potting soil or soil-less rooting medium.

For a good soil-less mix, blend equal parts by volume of LeafGro, peat moss and perlite. For every gallon of peat moss, add two heaping tablespoons of dolomitic limestone. Peat moss is very dry; moisten it well during mixing. Store the unused rooting medium in a plastic bag so it will remain moist.

To make potting soil, mix equal parts by volume garden soil, compost from your garden or commercial compost and perlite. Place the blend in a microwaveable container and microwave at full power for 15 minutes for each gallon of potting soil. Cool before using.

Unless the garden soil is very sandy, potting soils with more than one-third garden soil should never be used in glazed or plastic pots. Because soil is less porous and holds more water, potting media rich in garden soil should be used only in porous clay pots.

Potting soils that contain garden soil do not need to be watered or fertilized as often as those grown in soil-less media. Nor do they shrink as rapidly as those made mostly of peat moss and perlite or vermiculite.

Use soil-less rooting media in porous clay pots, and your plants will dry out more rapidly, thus requiring more frequent watering. More frequent watering means having to fertilize your plants more often.

You will always experiences better results growing your houseplants in a similar potting soil and containers.

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

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