The plant clinic gets this question every year. A mophead hydrangea (Hydrangea macrophylla) is growing like gangbusters, but nary a bloom in sight. Or the flowers are the wrong color.
The typical problem is soil chemistry: too much nitrogen, not enough phosphate, and/or the wrong pH. The best way to find out what is wrong is a soil test. The test results will tell you exactly what the problem is, and how to fix it.
But some problems are obvious even without a soil test.
If flowers on a supposedly blue hydrangea are pink, this is an indication of high pH. Most plants like their soil slightly acidic (pH below 7), but hydrangeas like it even more so (5.5 or below). Use 30 percent sulfur (also called “soil acidifier”) or 90 percent sulfur to get your blooms blue again. Using sulfur is better than acid-loving plant fertilizers, because the latter can also contain a lot of nitrogen (see below).
Sulfur will lower the pH more gradually, and keep it there longer. Thirty percent or 90 percent sulfur is much easier to work with than pure sulfur that is extremely dusty.
If you have a lot of healthy foliage, but no blooms, a couple of things could be wrong.
1. Too much nitrogen can cause lush foliage at the expense of flowers. Some acid-loving plant foods contain ammonium sulfate that is high in nitrogen, and this can make the problem worse.
2. The soil might be deficient in phosphorus (P); this is a common problem in WNC. Phosphorus promotes flower production.
In either case above try superphosphate (0-18-0) or triple superphosphate (0-45-0) to get phosphorus levels up. If you haven’t fertilized recently, using a fertilizer with a high middle number like 5-10-5 or 15-30-15 will have the same effect.
No matter what chemicals you add to the soil, be sure to read, understand and follow the label directions. And remember that too much fertilizer often causes more problems than too little.
Soil test boxes and forms are available at the Extension Center on Raccoon Road.
Jim Janke is an Extension Master Gardener Volunteer in Haywood County. For more information, call the Haywood County Extension Center at 828-456-3575.