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Hortiscope: Use vermiculite after considering risks, benefits

Q: I have a very large Thanksgiving cactus plant in a tall pot. The blooms go lower than the bottom of the pot and the plant takes up the whole pot. I would like to split it in half or quarters. What is the safest way to do this?

A: It can be split into quarters, thirds or halves, but do so after it has completed flowering, which should be sometime in March or April. I would encourage you to take some cuttings from the plant to root before doing the dividing in case there is some sort of disaster. That probably won't happen, but it is best to cover all possibilities.

Q: Can you use vermiculite in garden soil and how much can you use?

A: Vermiculite use is questionable in garden soil because of the potential asbestos it may contain. I have worked with it in the past because I was naive to the presence of this carcinogen. However, I'm still alive and healthy.

Since asbestos was discovered to be a carcinogen, huge efforts have been launched to minimize human exposure to it, especially the asbestos that was used for insulation. At the time (late 1970s), vermiculite contained about 4 percent to 5 percent asbestos. The material that was used for insulation had up to 80 percent asbestos. With most of the industrial asbestos now taken out of the environment, the Environmental Protection Agency redirected its focus to the lesser concentrations, such as vermiculite.

The EPA determined that the amount of asbestos in vermiculite will not pose a potential risk to casual users. However, to further avoid exposure to any potential airborne asbestos, the EPA came up with several recommendations.

Use vermiculite only in outdoor situations and keep it damp when distributing or working with it to minimize dust particles becoming airborne. Avoid bringing vermiculite indoors on clothing.

Use a premixed potting soil because it has a lower vermiculite content and usually is moist. This should minimize the generation of vermiculite dust. Try to use other garden soil conditioning products, such as peat moss, compost, weathered sawdust or bark. All of that said, it is up to the user to determine if the risk outweighs the benefit of using vermiculite.

For me the risk outweighs the benefit because there are so many other available products that lack any human health hazard that can be used.

Q: The leaves on my three houseplants keep turning yellow. It's not that they are over-watered because I only give them 2 cups every two weeks. I put a tablespoon of Miracle-Gro in the water. The water I use is spring water I buy at a grocery store. What is the best soil to use for houseplants?

A: The problem could be drainage. If the containers are not freely draining, the water could be sitting in the base of the potting soil and causing anaerobic conditions.

If that isn't the case, then it could be that the light the plants are receiving is insufficient. If that is the case, you need to move them to a location where the plants can get better light or get a plant light that you can put on a timer so the plants get 12-plus hours of light.

As for the soil, any media that is designated for use as a houseplant soil is acceptable. You could use soil that is designated for use with African violets. This type of soil usually is high in organic matter. One other thing to note is that houseplants do not need fertilizer. Fertilizer only is needed if the plants are actively growing, which yours are not. Save the fertilizer until spring as new growth begins.

To contact Ron Smith for answers to your questions, write to Ron Smith, NDSU Department of Plant Sciences, Dept. 7670, Box 6050, Fargo, ND 58108-6050 or e-mail