Research done by Jim Midcap at the Center for Applied Nursery Research in Georgia helps answer questions about how to be sure you get the color you want when growing some of the new reblooming bigleaf hydrangea cultivars.

Jim was working with the cultivars 'David Ramsey,' 'Endless Summer' and 'Penny Mac' which were potted into 3 gal containers in September using a pinebark and sand mix (9:1, v/v). Their fertilizer was Osmocote 20-4-8 at 10 #/cu. yd. . . . there was 4 lbs of dolomitic limestone per cu. yd. as well. The following April, all plants were topdressed with the Osmocote at 57 g. plus one tablespoon of iron chelate/pot.

He had 3 treatments which were applied April 14 for the aluminum sulfate (1.5g per pot) and April 18 for the flowable lime (80 g per pot). Plants either got only aluminum sulfate, only lime or both aluminum sulfate and lime. Jim measured pH and flower color based on these treatments.

Results were dramatic! "The aluminum sulfate treatment decreased pH and produced the bluest flowers. (Flowers were present in late May.) The combination treatment of aluminum sulfate and lime increased pH while producing grape to purple flowers. The lime treatment alone always produced bright pink flowers since there is no aluminum in the pine bark mixes." Blue flowering plants averaged a pH of 3.67 by mid June. The purple flowered plants averaged a pH of 5.37 and the pink plants averaged a pH of 5.27.

Jim's "Significance to the Industry" is: "The application of aluminum to acid pine bark mixes will produce blue flowers on most Hydrangea macrophylla cultivars. The absence of aluminum in these mixes will produce pink to red flowers on all cultivars. The purple or grape colors can be obtained by applying aluminum and raising the pH to the 5.0 to 5.5 level."

From: J. Midcap. 2003. Flower Color Control on Remontant Flowering Hydrangea macrophylla cultivars. GGIA Jnournal Feb. 2003:27.

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North Carolina State University