Monday, August 2, 2010

Your Houseplants are Screaming: "You Call This Food?!?"

This was going to be a rant on houseplant fertilizers. I was about to dump on the folly of the latest marketing innovation in houseplant food: the drip feeder. 

These inexpensive (about $1.50), hypodermic-like IV's for houseplants only offer miniscule amounts of nitrogen, phosphorus and potassium, and only for a short time, about two weeks. On the plus side when it comes to houseplants, these mini-meal drippers (which put your plants on a less than 1% N-P-K diet...wayyyyy less than 1%) are healthier that the big buffet offerings of a typical, water soluble plant food that may offer up to 20% N-P-K. 

Huge, salt-based meals like that for houseplants usually result in weak growth, root rot, leaf burn, salt deposits and the dropping of lower leaves.

The big problem with these one-ounce fertilizer drippers, which are intended to be used in an eight-inch pot? All the fertilizer is going to a very small area of the plant roots. One ounce of a liquid, dripping out at the rate of a few drops a day, is not going to percolate throughout the entire rootball. Yes, watering the soil before adding the dripper (as recommended on the label) will help spread the fertilizer. But not enough to nourish all the roots. Or even reach the bottom of the container. And the few roots the liquid does reach? Root burn could still occur, since it is dripping in a very small area. 

And seeing how the N-P-K content of these dripper products is measured in the hundredths and thousandths of one percent, you may be providing more nutrients by "feeding" your houseplants...tap water!

Texas A&M University advises to go easy on the fertilizer, following label directions. "When applying fertilizer in a solution, make sure that some runs out of the bottom of the pot. This prevents root burn and the buildup of soluble salts or excess fertilizer and reduces the chance of burning the plant."
An Ohio State University report concludes that the best way to fertilize houseplants is to... "fertilize at half the recommended strength, every 2 weeks from March to September. Do not fertilize most plants during winter months. Winter's reduced light and temperature result in little or no growth. Most houseplants are dormant during winter."

But, as always, the more I researched the topic, the more I realized there is a lot more to learn, including this surprising tidbit from the Horticulture Department of North Carolina State University
"Houseplants grown under less than 200 foot candles (a measurement of light intensity) benefit little from fertilization and may actually be harmed." 

Yes, there are good arguments for NOT fertilizing houseplants in low light situations. And your house, if typical, is probably well below that 200 foot-candle threshold. You may be slowly killing your houseplants with too much food and too little light.

Next time: how to measure the amount of light in your house, using ordinary household items. And, choosing houseplants that do best in low light conditions.

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