March 19

1 - Proper houseplant watering compensates for shrinkage of potting soil.

Q. Your Yard and Garden post of 12 February mentioned "proper irrigation practices" for indoor houseplants. Please explain this. Inquiring readers want to know!

John C. Mc.


A. Thank you for asking. I often have too little room in my column to explain some topics, so I count on readers asking for more detail. I was hoping someone would catch that phrase. Proper irrigation is based on several factors. First, is the plant being grown have its water requirements and its need for drying between irrigations met? It also depends on the quality of the water being used, and finally on the potting soil and time since last repotting. I will not have time to discuss water requirements for specific houseplants, but some need to be kept consistently moist, while others benefit from drying between watering. All plant roots need oxygen in the pore spaces between soil particles, so it is good to match both potting soil and irrigation to the needs of the plants. Those plants that need drying require higher levels of oxygen around their roots and often benefit from a potting soil that drains rapidly. However, potting soils often shrink when they dry. This creates a problem for the next irrigation. Water will often run around the soil rather than moistening the soil. Gardeners are told to water until water runs out the drainage hole in the bottom of the pot, but in the case of soils that shrink; this will not provide adequate water. Slow watering may help moisten the soil, but a faster way is to water from the bottom of the pot. Place the pot in a dish pan with enough water to cover the base of the pot, but not enough to cause the pot and soil to float (and turn over). Allow the water to soak up from the bottom to moisten the soil. This violates the gardening rule that says not to let potted plants sit in water. This rule is to prevent water logging the root system and causing it to rot and to prevent accumulation of mineral salts in the soil. To accommodate these purposes, do not leave the pots in the dishpan of water for a prolonged time (more than a few hours). Once the soil has moistened, remove the pot from the potted plant from the water and allow surplus water to drain away. By moistening the soil well, these accumulated salts will dissolve and more easily be drained from the soil. Of course, this also removes fertilizer salts, so you will need to fertilize periodically to replace the lost nutrients. A potting soil that has been in use for several months to years may have significant quantities of minerals that have accumulated. The quality of the irrigation water (the concentration of dissolved minerals) determines when too much mineral salt has accumulated and may begin causing "salt burn" symptoms in the plant leaves. Irrigation by dissolving and draining away surplus salts will help reduce the problem and irrigating with rainwater will help even more. Eventually, it will become necessary to repot the plants to replace salt laden soil with fresh soil. Now that spring approaches and warmer weather is in the near future, you can begin planning for repotting. When repotting plants with significant salt accumulation, it is wise to repot outside where you can was much of the old soil from the roots. This will also wash away some of the accumulated minerals. Then repot and replace the soil that was washed away. Water well to settle the soil around the roots and begin the "proper irrigation practices" that will slow the accumulation of salts. Yes, use rainwater whenever possible.

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:, office: 505-865-7340, ext. 113.


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