REPOTTING A CHRISTMAS CACTUS

by Curtis Smith, Ph.D.  

Q. I am ready to re-pot my Christmas cactus and am wondering what the proper soil is for it. Would it be best to use a potting soil such as Miracle-Gro, or would it need a more acidic soil such as that for African violets?

A. Now is a good time to repot Christmas cacti (Schlumbergera spp.). Following flowering they should have begun growing, or will begin soon. The soil into which you transplant them is important, but the Christmas cactus is tolerant of various conditions. It is very important that the potting soil be well drained. This is one of the plants that will not tolerate soggy conditions - it needs air around its roots. Having said that, I have seen Christmas cacti growing very well in garden soil used as potting soil.

The ideal potting soil will contain a high proportion of organic matter such as peat or well-decomposed compost. Addition of coarse sand, or even aquarium gravel, will provide the necessary drainage and aeration. There are commercially available cactus potting soils that are designed to be well-drained soils.

The reason for such a need for aeration is that these plants are native to tropical rain-forests where they grow as epiphytes high in the trees. The "soil" in these locations is just composted tree leaves and whatever else gets caught in this "compost pile." There is a distinct dry season, but even in the wet season when it rains daily, the location of these plants high in the trees causes drying of the compost around the roots each day. Roots in a pot will not dry as completely, but a well-drained potting soil allows adequate aeration for healthy root growth.

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Q. Can I make my own potting soil by taking soil from the garden?

A. Garden soil works best in the garden but can be used in potting soil. In the confined environment of a pot, addition of certain amendments is helpful. Compost, peat, or other organic matter is a good addition to garden soil. If the garden soil is quite sandy, organic matter may be all that is required. If the garden soil has a lot of clay, perhaps some coarser material such as coarse sand or aquarium gravel should be added, depending on the plants to be grown.

Another consideration is the presence of fungi and bacteria in all garden soils. In the confined environment of a pot, these are more likely to damage plants. You can pasteurize the soil by heating it to a temperature of 160 degrees for 30 minutes. Heated soil does not smell good, so if you can pasteurize the soil outside, it is advisable. You can use a portable oven or barbecue for outdoor soil pasteurization.

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Q. What exactly is meant by 'as soon as ground can be worked in spring'?

A. Thanks for this question. Jargon can sometimes get us in trouble, but it can also be a teaching point. For you, in Las Cruces, the phrase "when the ground can be worked in the spring" probably has no meaning. It refers to soil being too cold -- the water in the soil frozen -- or too wet, which is an important consideration for gardeners with clay soils in moister environments. Frozen soil is difficult or impossible to "work," ie. to turn it with a shovel or rototiller. I have tried using a pick on frozen soil, but it's a lot easier to wait until the soil thaws. Shady areas of the garden on the north side of my house in Albuquerque are very unworkable right now. Clay soils present unique problems because of their ability to hold water. If the soil is a wet clay it is not good for the soil to turn it. When it is dry it is also difficult to work.

In your area, dry soil is more often the problem, and can be remedied by irrigating and then waiting for a few days. In moister climates, clay is often too wet in the spring to be easily turned. Clay has a very narrow moisture range when it can be successfully rototilled or turned with a shovel. Addition of organic matter helps with the moisture problem, but the soil is still difficult to work when frozen.

So, for you, this term is of little relevance, but for other parts of New Mexico and much of the U.S., soil workability is an important consideration

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Q. How can I tell if I have enough rain or snow so that I don't need to irrigate in the winter? I don't want to waste water, but I don't want my plants injured either.

A. The only way to know for sure is to probe the soil by digging or using a soil probe to determine the depth of moist soil. However, there are some general guidelines you can use. Sandy soil moistens more deeply than silt or clay even with the same amount of moisture. Silt also moistens more deeply than clay. For example, one inch of rainfall (roughly 10 inches of snow) will moisten sandy soil to a depth of one foot, while moistening silt to six inches and clay to three inches. If the rain comes rapidly and runs off onto surrounding land or the street, then the soil will moisten less deeply. Snow may sit on the soil and slowly sublimate, or evaporate. In this case, you again have less soil moistened. In colder areas of our state, snow often sits on frozen soil, which cannot absorb moisture, so even more may be lost without benefiting the soil if the ground is frozen.

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Send your gardening questions to:
Yard and Garden, ATTN: Dr. Curtis Smith
NMSU Cooperative Extension Service
9301 Indian School Road, NE, Suite 112
Albuquerque, NM 87112

Curtis W. Smith, Ph.D., is a an Extension Horticulture Specialist with New Mexico State University's Cooperative Extension Service. Dr. Smith and Dr. George Dickerson host Southwest Yard & Garden, a weekly program for gardeners in the Southwest. It airs on KRWG in Las Cruces Saturdays at 11:30 a.m., repeating Thursdays at 1 p.m.; on KENW in Portales on Saturdays at 10 a.m.; and on KNME in Albuquerque on Saturdays at 9:30 a.m.