Don Kinzler, Published January 31 2014
Growing Together: Prescriptions for sick houseplants
When plants get the flu, we need to be part detective and part doctor to avoid calling the coroner post mortem.
In some ways diagnosing houseplant troubles might be more challenging than diagnosing human health issues. All humans belong to one scientific species. But there are hundreds of houseplant species, each exhibiting various aches and pains when not feeling well.
Plants are like humans. The best remedy for sickness is prevention, and the most common cause of houseplant failure is improper care or environment. This is good news because these are things we can control.
There are relatively few diseases and insects lurking in our homes waiting to attack our plants. Most problems are cultural.
Improper watering is the leading cause of plant illness. To prevent problems, let’s examine the best watering procedure.
Deciding how much to apply is easy. Each time a plant is watered, add enough to thoroughly wet all the soil in the pot. Then discard any excess water that collects in the bottom drainage dish immediately before it soaks back up.
This drainage water can contain salts that have “leached” out of the soil, so don’t dump it on another plant either.
Knowing how often to water is tricky. Small pots usually need watering more often than large pots. Sun-loving plants in a bright window dry out faster than low-light plants in the corner of a room. Warm rooms and locations near heaters or air ducts increase water demand.
Some plant types such as ferns and African violets prefer more frequent uniform watering than succulent-leaved species like jade plant which prefer thorough drying. Plants often need more frequent watering in winter when the furnace is running and humidity is lower than during humid summer weather.
Because of these factors, it’s difficult to set a one-size-fits-all watering calendar. Published literature often recommends watering when soil is “dry to the touch,” which isn’t always a clear description.
Poke a finger into the soil 1 or 2 inches. If the top inch of soil feels dry, but you can feel moisture below that, it’s probably the right time to water. If there’s no moisture below, you’ve waited a bit long. If the soil feels wet and sticky below the top inch, it might be staying too moist.
Next let’s ask our plants to stick out their tongues and say “aahh” so we can discuss six common symptoms and causes.
1. Wilting of leaves can be caused obviously by underwatering, but it can also be a symptom of overwatering, as the roots gasp for oxygen or succumb to rot.
If you suspect overwatering, examine the roots. Healthy roots are white or cream colored. Rotting roots are brownish black and sometimes slimy.
Repot immediately into new quality potting mix.
2. Yellowing of lower leaves and eventually dropping can signal a root-bound plant in need of repotting or fertilizing. Some leaf drop is normal, especially as plants age.
3. If all leaves turn yellow, suspect root rot. Other causes are improper light, temperature, fertilizer problems, and insects.
4. Browning of leaf tips or margins is frequently caused by hot, dry air and low humidity.
It can also signal a buildup of salts such as fluoride in the soil. Occasional flushing of the soil “leaches” out salts. Allow chlorinated water to stand overnight to dissipate. Water from a softener should be avoided. Certain species, such as spider plants and peace lily, are very susceptible to tip burn.
5. Sudden loss of leaves is caused by rapid changes in temperature or moving a plant from a sunny to a dim location. Exposure to a quick draft of hot or cold air can trigger leaf drop.
6. Mottled, light green leaves with pin-sized specks can indicate a spider mite infestation. Shake a leaf over white paper. If the small specks move, they’re probably mites, which are often undiagnosed because of their size. Treat with insecticidal soap or Neem oil.
If plants are looking sickly from one or more of these symptoms, “feeding” with fertilizer usually won’t help. Fertilizer won’t cure problems caused by insects, improper watering, poor drainage or low light. These need to be remedied separately.
Fertilizer is for healthy houseplants. A good rule of thumb is to fertilize once a month during periods of active growth in spring and summer, and only once or twice during the entire winter. An exception is flowering plants.
Do I talk to my houseplants when they’re ailing? It depends on the definition of “talk.” If it means a communication and understanding of the care needed by each plant, then yes, I talk to my plants. But let’s keep that between you and me.
Don Kinzler, a lifelong gardener, worked as an NDSU Extension horticulturist and owned Kinzler’s Greenhouse in Fargo. Readers can reach him at firstname.lastname@example.org