Watering potted plants from top or bottom

I'm puzzled. I heard that some plants should be watered from above and others from below. Which plants are in which category?

Answer:

Many people think that they should water African violets and some other hairy-leafed plants from below to prevent spotting of the leaves. I have not had a problem with this in any of my African violets, perhaps because our very dry air allows drying before disease has a chance to develop. I think this is one of the gardening concerns brought to the Southwest from other regions where growing conditions are different.

Here in the southwestern U.S., we have different concerns. One of the primary concerns when growing potted plants is the accumulation of salts in the soil. These salts are not the familiar table salt (sodium chloride), although that salt is possible. The salts we are concerned with are more likely to be compounds containing calcium, which is present in our well waters and our southwestern soils. Accumulation of salts can cause root damage and death of the plant. Irrigating potted plants from below can cause rapid accumulation of salts in the pot. However, improper irrigation from above can also cause salt accumulation problems.

There are instances when watering from below is beneficial. Soaking from below is especially helpful in wetting a soil that has dried so much that it does not absorb water applied from above. Such soils often have cracks that allow water to flow through quickly without wetting the soil, or the soil has pulled away from the side of the pot, allowing water to run around the soil without wetting it.

To prevent accumulation of salt, water must drain from soil after irrigation to "leach" away dissolved salts. This is true whether the plant is watered from above or below. Gardeners often place a dish under the flower pot to protect furniture from leachate water, but if this water is allowed to re-absorb into the pot, the leached salts accumulate in the soil. Place the bottom of the pot above the level of accumulated leachate water by adding a layer of gravel to the dish under the pot to prevent re-absorption of water, or pour away the leachate after watering.

If it is necessary to water from below to moisten dry soil, allow surplus water to drain away after the soil has moistened. This can be done by removing the pot from the container of water and placing it over a drain rack in an empty container or by placing the pot into a sink or bathtub so that the water freely drains away.

Late winter and early spring is a time when problems associated with accumulation of salts in pots of houseplants becomes evident. After a few months of indoor watering, as days lengthen and temperatures rise, damage begins to appear when plants begin new growth. If the soil has accumulated salt, it is wise to repot the plant in new potting soil. Replace much of the old potting soil at this time. After repotting, begin watering as described above.

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Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the Desert Blooms Facebook.

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