Gardening with Allen: Indoor plants benefit from fertilizer boost in winter

Some of my indoor plant leaves are turning yellow. Do I need to fertilize them? What kind of fertilizer do you recommend?

How long has it been since you fed your indoor plants? Because of lower light levels and shorter days, indoor plants grow more slowly during winter months and therefore use less fertilizer. However, plants do need regular fertilization for best growth.

The best indication of need for additional fertilizer is older leaves turning light green and yellow. An occasional yellow leaf is not cause for concern. Plants naturally drop the oldest leaves as they mature and are no longer producing food. Certain nutrients are transferred from old leaves to new growth in a natural recycling process. Nitrogen is the main nutrient transferred. However, if nitrogen supply in inadequate, it will be transferred from older leaves in order to keep up with new growth.

Iron and certain other micro-nutrients cannot be transferred from old to new growth. Therefore, if they are in short supply, the new leaves start to yellow, but continue to have green veins. If newest growth turns light green to yellow without green veins, it may be caused by excess fertilizer or salt accumulation caused by watering with softened water.

Lack of adequate light, cold temperature, acid soil and excess watering can also cause plant leaves to turn yellow. So consider these factors when deciding what action to take.

I use an indoor plant fertilizer which contains both nitrogen and iron as a way to avoid both causes of yellowing. Liquid and soluble powder fertilizers are convenient to apply when plants are irrigated. There are two rates for most fertilizers. The lower rate allows you to add a little fertilizer every time you water. This applies fertilizer according to plant needs and is the easiest to remember. Since plants use less water in the winter, fertilizer is also reduced. A higher rate is usually given for monthly or less frequent irrigation. If you use this method, mark your calendar so you won’t forget. You may want to lengthen the interval between fertilizations by a week or two during winter.

My favorite type of indoor fertilizer is time release or slow release. Fertilizer pellets are covered with a plastic coating which contains tiny, microscopic pores. Every time a pot is watered, some of the water flows in through these pores, dissolves a little fertilizer and flows out again. These encapsulated fertilizers last from 3 to 6 months. I like to make a calendar note based upon the fertilizer label so I will know when it is time to reapply. I also watch for signs of yellowing leaves and apply some extra liquid fertilizer as needed.

Allen Wilson is a Vancouver gardening specialist. Email Allen Wilson at allenw98663@yahoo.com.

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