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Is your phlox receiving enough sun, or is it growing in heavy shade? Have you properly prepared the soil before planting the phlox? Phlox needs a rich, well-drained, moist soil to grow well. The sub-layer of soil below the topsoil layer should be loosened up with a spade and have some coarse sand and peat moss added. Then, a 3 or 4 inch layer of organic matter such as well-rotted manure, peat moss or compost, as well as some coarse sand and bonemeal should be added to the topsoil and thoroughly worked in. Bonemeal is a good source of phosphorous and should be worked into the soil prior to planting so it is readily available to the plants roots.
This is important because phosphorous moves so slowly in the soil, unlike nitrogen and potassium.
In any case, the magic fertilizer for perennials is one such as 0-20-20, or any of those with a low nitrogen formula. Perennials that have too much nitrogen produce weak, succulent stems and large, over-grown leaves rather than flowers. The middle #20 - phosphorous, promotes strong roots, stems and blossoms as well as the rich color of the flowers and foliage. The second #20 is potassium - the nutrient that helps plants resist diseases and cold, among other functions. Of course, a plant must have some nitrogen in order to grow. A plant that lacks nitrogen will have stunted and spindly stems, small and erect leaves, pre-mature leafdrop and eventual chlorosis - the entire plant will probably appear a lighter green than normal.
I find it best to use a water soluble fertilizer when fertilizing the plants. The water carries the nutrients directly down to the roots where it is needed. Apply the fertilizer at the rate recommended by the manufacturer. There are cases where the soil may also lack micro-nutrients. A good fertilizer in this case is water soluble Miracle-Gro 15-30-15 which has several micro-nutrients (trace minerals) added to it.
I hope your phlox will out perform all others this year. Best of luck.
