TIA

Since you should have on hand a 3:1:2 ratio fertilizer for almost all your container gardening needs anyway, it's quite easy to turn your 3:1:2 ratio fertilizer into a 3:1:3 ratio by adding a little potash or Dyna-Gro's Pro-TeKt 0-0-3 (better choice because it has silicon in it) to your fertilizer solution when you fertilize.

It's best to avoid 1:1:1 ratio fertilizers like 20-20-20 or 14-14-14 because they supply much too much P in relation to N or K; and it's important to avoid all bloom-booster fertilizers and fertilizers with a middle number as high as or higher than either the first or third numbers (N or K).

In the end, you can use any 3:1:2 ratio fertilizer (I like Foliage-Pro 9-3-6 best) right from the package with good results, but adding a little extra K to that strategy would be an improvement and please the plant.

Al

Taken from an abstract in Hort Science (I've edited it to make it more readable):

A series of experiments were undertaken to determine the effects of nitrogen (N), phosphorus (P), and potassium (K) concentrations and N:P:K ratio on flowering and vegetative growth of two herbaceous perennials, Hibiscus moscheutos L. (hibiscus) and Rudbeckia fulgida var. sullivantii Ait. 'Goldsturm' (rudbeckia). Plant growth and flowering of both hibiscus and rudbeckia were influenced by concentration and ratio of N, P, and K. When N was held constant at 100 mg/L, 4:1 N:K and 16:1 N:P were optimal for growing hibiscus [that's 16 times more N than P], whereas higher K concentration and lower P concentration were required for optimal growth of rudbeckia. However, when holding N constant at 100 mg/l and varying both P and K in the fertilizer solutions, higher P and K concentrations and a 2:1:2 N:P:K ratio best supported hibiscus growth, whereas 3:1:2 N:P:K was needed for growth of rudbeckia. Finally, when both N concentration and N:P:K ratio were altered, optimum growth of both hibiscus and rudbeckia was achieved at similar and lower P and K concentrations - an 8:1:2 N:P:K ratio was optimum for production of both hibiscus and rudbeckia, although 12:1:2 N:P:K produced similar growth of rudbeckia.

The short answer is, the higher levels of P don't encourage blooming in hibiscus because they are being offered at toxic levels. Also, it's much more effective in almost all cases to reduce the N supply instead of increasing P to promote blooming. Reducing N curtails vegetative growth, which forces the plant to spend it's energy on producing reproductive tissues instead of vegetative, so the result is more blooms/fruit.

Assuming you have the right light/temp, the keys to bringing along great hibiscus in pots are - an appropriate soil, YEARLY repotting (not just potting up), and an appropriate fertilizer - one that is LOW in P.

Al

Let me know if you have any questions.

Al