Answer: As you know, Thrift (Armeria) is a perennial plant, and perennials don't necessarily live forever. Thrift need extremely well-drained soil, like full sun, and little to moderate water. They can be propagated by dividing the plants in spring or fall, or by saving seed and sowing it in spring. They also benefit from fertilizer in spring, just as new growth begins.
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
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