The Q&A Archives: Yellowing Leaves On Petunias

Question: My potted petunias have been beautiful all summer until recently when the leaves started to turn yellow and die from the bottom up. While the leaves were turning yellow, the plants were still flowering nicely. What is causing this? How can I prevent it? Will cutting the plants way back help? I have been pinching and trimming them back all summer.

Answer: Yellow leaves can be caused by many things including lack of nitrogen, insufficient light, water-logged soil (plant roots need oxygen to thrive), dry soil, or iron deficiency. If the older bottom leaves are yellow, but new growth is green, it's usually a lack of nitrogen. If new leaves are yellow, with green veins, it's usually a lack of iron. (Lack of nitrogen is a more common problem than lack of iron.) Soil should be kept moderately moist (but not wet).  Finally, transplant shock can contribute to yellowing.

You didn't mention if you fertilize your pots. Since your plants have been performing all summer, they may have used up all the available nitrogen in the soil. Here?s some basic info on fertilizer and nutrients that plants require.  The 3 numbers on a fertilizer bag refer to the percentage of N (nitrogen), P (phosphorous), and K (potassium) in the bag. There are different formulations for different purposes. In general terms, nitrogen produces lush green growth, phosphorous helps strengthen stems and produce flowers, and potassium keeps the root system healthy. If you're applying fertilizer to fruiting (e.g., tomatoes) or flowering plants, you're not as interested in the plant developing leaves as you are in it flowers and fruit, so you'd use a formulation lower in nitrogen and higher in phosphorous, such as 15-30-15.  Bone meal is an organic source of phosphorous.

Organic sources of nutrients:

Nitrogen:  alfalfa meal, blood meal, coffee grounds, cottonseed meal, fish emulsion, seabird guano.
Phosphorous: bone meal, rock phosphate
Potassium: greensand, seaweed, kelp

I suggest you apply some fertilizer. Pinching them back won't affect the yellowing.

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