One aspect of houseplant care that is often neglected is leaching. It simply means pouring water through soil to dissolve and flush out undesirable minerals. And no leeches (note the different spelling: leech refers to a bloodsucking worm; leach to a watering technique) are required.
Leaching is often used outdoors to treat soil contaminated by chemicals, notably, in early spring in cold climates to flush road salt from lawns located near salt-treated roads. But it can—and should!—be used on plants grown indoors. That’s because, since a houseplant is watered in a closed environment (a saucer collects surplus drainage water which is then reabsorbed by the plant, so any wayward salts never drain away), excess mineral salts (calcium, sodium, potassium, phosphorus, etc.) slowly accumulate in the soil as the months go by.
Do note that these salts are naturally present in tap water as well as in fertilizers and, in small quantities, are beneficial to plants. But when they accumulate in the soil, they start to “burn” the plant’s roots (kill them starting at the root tips). Hard water (and municipal tap water and well water are frequently hard) contains the greatest concentration of salts and plants watered with either will need fairly frequent leaching. Distilled water and rainwater are free of mineral salts … but plants watered with either will still eventually need leaching if, as is usually the case, you’ve been adding fertilizers to the soil.
Crustiness: Not Next to Godliness
When you see a white or yellowish crust form on the inner wall of the pot or, worse yet, on the stem of the plant, that’s a sign that leaching is overdue. At this point, excess salts have already started killing roots. It’s better to begin leaching well before reaching that point.
The best treatment is to simply make a habit of leaching 2 or 3 times a year. If your tap water is very hard, with a pH of 7.5 or more (check with your municipality), leaching every two months is not excessive. If the plant spends the summer outside in a climate where rain is frequent, it will take care of leaching during that season.
Some plants are less tolerant of accumulated mineral salts than others (corn plants, spider plants, azaleas, etc.) and may need monthly leaching.
To leach a plant, simply remove its saucer and place it in the sink, the bathtub or outdoors. Water it normally, moistening the root ball as usual. Then wait 5 minutes (this will give the salts in the soil time to dissolve), then water again, as abundantly as the first time.
Excess water, now rich in salts, will flow from the pot through its drainage holes and disappear down the drain or into the ground. This will lower the concentration of salts in the pot, giving your plant a new lease on life!
Don’t Forget to Repot
In a sense, leaching is a stopgap method, keeping the potting soil in decent shape for a longer period. However, over time, all container soils degrade, becoming more mineralized, denser, less aerated … and less convivial to healthy plant growth. So, do consider repotting occasionally. With most houseplants, repotting every two to three years is quite reasonable.