Are Your Indoor Plants Hungry?

My indoor plants are starting to develop light green and yellow leaves. Do you think they need fertilizer? What fertilizer do you recommend for indoor plants?

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 more of the older leaves causing them to turn yellow.

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, alkaline soil, and excess watering can also cause plant leaves to turn light green and 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 months.

My favorite type of indoor fertilizer is time release or slow release. Osmocote is one of the most common brands. 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. This method also applies fertilizer according to plant needs. These encapsulated fertilizers last from 3 to 4 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

Allen has been writing about gardening for over 30 years. He is a retired professor of Horticulture.

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