Question: Should houseplants be fertilized in the winter? 

Answer: Most houseplant ailments relate to over watering, coupled with bad fertilizing practices. It is surprising how many people I know say, “My houseplants look sick; they never bloom.” But when I ask them if they fertilize, they say, “No.” 

Here are some of the basic ground rules for houseplant fertilization. 

With houseplants and fertilizer, it is best to err on the side of too little, than too much. Houseplants do not need to be fertilized much this time of year (winter). Most plants do best with a yearly rest period — winter gives them a needed break. They need plant food most when they are actively growing.  

I usually stop fertilizing my plants in November, then start up again with a very low dose (diluted) of fertilizer again in the new year (now) when the days are slowly getting longer. Spring is a good time to start fertilizing houseplants with slightly higher doses of fertilizer for bloom and growth. 

Mixing directions on labels for commercial houseplant fertilizer often are too strong and too often for most houseplants. Be skeptical. Use a more diluted solution than recommended, and see if that works. Too much fertilizer will burn the plant and ruin the soil of a houseplant. 

MORE: Read past garden advice columns from Carol Savonen. 

Frequency of fertilizer application will vary, depending on the time of year, the type of plants you have, the vigor of growth you want and the age of each plant. Some plants do best with fertilization every two weeks, while others will flower well for several months without needing any supplement. Plants grown in peat moss and vermiculite or perlite need fertilization more often than those grown in a fertile potting mix.       Again, I’ll stress that the key to success is moderation. Feed your indoor plants sparingly and ‘double water’ monthly to flush away excess fertilizer salts. To double water, water once as usual, then follow with another watering five minutes later with enough water to cause some to run out the bottom of the pot. Rainwater is great for watering houseplants; it prevents a salty buildup of minerals in your houseplant soil. 

More:Why rain is better than tap water for houseplants

Houseplants need fertilizers containing three major elements: nitrogen (N), phosphorus (P, sometimes listed as P2O5) and potassium (K, sometimes listed as K2O or potash).       Sometimes called “plant food,” houseplant fertilizer is sold in granular, crystalline and liquid or tablet forms, under a multitude of brand names. The label should indicate how much water-soluble nitrogen, phosphate or potassium is available per pound of product. Organic houseplant fertilizers are available as well.       Pay attention to the numbers following the fertilizer name on fertilizer packages. These numbers correspond to the percentage of the nutrients — N, P (P2O5) and K (potash) by weight in the fertilizer. 

A “well balanced” fertilizer means that there are equal amounts of each of these three major nutrients.  For example, a 20-20-20 mixture is well-balanced and is an optimal ration for feeding leafy plants without a lot of blooms.   To encourage copious blooms on a plant, phosphorus is key. You’ll want to use a fertilizer with a higher amount of phosphorus, say a 10-20-10 ratio.  Fertilizer mixtures can be used to stimulate growth or enhance flowering or to simply maintain the plant.      Many houseplant fertilizers are rated about 20-20-20. If the numbers don’t add up to 100, it means there are inert ingredients or other nutrients in the fertilizer. Sometimes there is a fourth number, which is used for sulfur, another plant nutrient.

Besides N-P-K, many houseplant fertilizers also contain “micronutrients,” other elements that plants require in teeny-tiny amounts, including cobalt, copper, boron, manganese, molybdenum and zinc. 

Location matters when choosing houseplants

If you grow indoor ferns, fish fertilizers are the ticket. Liquid fish emulsion fertilizers are organic and are often less concentrated than chemical formulas such as “Miracle-Gro.” Look for “deoderized” on the label of fish fertilizers, as the stuff they sell for outdoor use is stinky. 

Some plants, such as orchids do best with a fertilizer formulated for acid-loving plants. When repotting plants or planting new offshoots into new pots, I like to use a kelp -based fertilizer to promote root growth. 

Carol Savonen is a naturalist and writer. She is an associate professor emeritus at OSU and tends a large garden in the Coast Range Hills west of Philomath with her husband and dogs. She can be reached at Carol.Savonen@oregonstate.edu or c/o: EESC, 422 Kerr Admin. Bldg., OSU, Corvallis, OR 97331.

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