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Be sure to fertilize well. I find my plants do best with compost. I use manure compost from my nephew's clydsedale farm. My flower gardens and vegetable beds do tons better with this rich mixture, and it's an eco friendly, natural fertilizer. Be sure if you use manure compost that it has been sitting for a couple of years so it won't burn your plants. My nephew keeps his out in a big pile so the rains and weather conditions can "cool" it off. "Hot" compost will feel warm & may even steam, if applied hot, it burn & may even kill plants and bushes. Put it on the garden areas in the spring early, before planting or fall after cutting back plants. If applied in the fall, the winter snows can leach nutrients into the ground all winter with the melting early in the fall & early in the spring. As the compost breaks down it feeds the soil. Carefully, working the ground around the plant well, digging and turning the soil breaking it up well and mixing in the compost will allow nutrients to get to the roots of the plants and bushes quicker & revive ailing plants more readily & completely.
Always be sure to provide adequate, consistent water & keep in partially shaded area. During the heat of summer water more frequently. Watch plants & bushes for signs of "thirst" such as drooping learves and blooms. Hydrangeas that have been overheated usually pop back within a few hours once they get watered. Soak them well every couple of days more often if they droop severely. This lessens the stress on the plant allowing it to develop more beautifully rather than just fight to survive. It can grow larger and bloom more abundantly.
Adding fertilizers purchased at the garden center can help produce more blooms too. Read the labels and look for proper bloomer fertilizers. These should have proper amounts of N-K-P.
Nitrogen, Potassium, Phoshrus.
Nitrogen (N) produces greener heathlier leaf system.
Potassium (K)produces more blooms & helps with photosynthesis.
Phosphorus (P) help with root system strength & tugor of the plant, and photosynthesis.
So, the more important nutrient for more blooms would be (K) Potssium. Follow directions as to amount and frequency of feeding the plants.