Phosphorus fertilizer for hydrangeas?

Asked April 17, 2019, 8:50 AM EDT

Should I use a heavy phosphorus fertilizer to help my hydrangeas bloom? I have 4 hydrangeas that are about 8 years old. They were originally planted in a very shady area and, after a couple of years of good blooms, the plants grew great foliage but only had 1 or two blooms per plant. 18 months ago I moved all 4 plants to the east wall of my house where they get full sun. I believe I fertilized them in the spring with a sprinkling of generic 12-12-12 but the plants still failed to bloom. I assumed it might be due to the transplant, or the after effects of yet another harsh winter. This year I was advised at a local nursery to fertilize with a 0-18-0 to really push the flowering. At the same time, I've heard a few researchers (such as Linda Chalker-Scott) really frown on the overuse of phosphorus in the garden.

My understanding is that the location is now ideal for these plants (morning sun, afternoon shade). I'm torn between 3 solutions that seem diametrically opposed to one another. Should I go the completely organic rout by spreading compost and then overlaying it with mulch? Should I continue with my past practice and sprinkle either a season long slow release chemical fertilizer or a multi application general fertilizer such as 12-12-12? Or should I follow the advise from the nursery and lean in heavily with a 0-18-0 fertilizer. Or, is the solution "None of the above" ;-)

I really try to go organic when I can, and I have listened to over 20 hours of lectures from Dr. Chalker-Scott, so I believe it when she says you shouldn't add anything to your soil without getting a soil test first (being done this weekend), but is there a general best practice that stands out to you as a path forward?

P.S. I haven’t pruned these in 4 years.

Washtenaw County Michigan

1 Response

When you fertilize plants you should use a fertilizer meant for specific plants. Turf is often a fertilizer such as 20-0-5 because nitrogen adds to vegetative growth. Trees and shrubs have a fertilizer analysis ratio around 3-1-1 or 3-1-2. Flowering shrubs may need a fertilizer meant more for flowering than growth of the plant with higher rates of phosphorus. But it should be based on what is already available in the soil. You may have enough phosphorus already but the growth with no flowers may be due to using higher amount of nitrogen than needed. It is recommended that you start with a soil test to get a basis of available nutrients and from there we can recommend a fertilizer. Some Hydrangea also only bloom on old wood from last season and if they are pruned back at the wrong time or have winter injury flower buds may have been pruned out or killed by cold. Do you know the type of hydrangea you have? A soil test can be obtained through the Washtenaw MSU Extension office. Call 734 997-1678 to check for their availability.