I think just about all of us as Master Gardeners have been asked why an Endless Summer Hydrangea didn't bloom -and how to get it to bloom in the coming year and produce blue-colored blooms. I just got off the phone with a home gardener who asked this very question. I spoke to my friend and colleague, and hydrangea expert, Kathy Zuzek, and went right to Bailey Nursery (Endless Summer breeder) to see what they had to say. Here is some info that may help you, as a Master Gardener, to answer this question down the road:
1. Too much nitrogen. Nitrogen causes plants to produce foliage. Excessive nitrogen fertilizer can cause a E.S. Hydrangea to grow more leaves, but not bloom. Fertilize a higher persentage of phosphorous (the second number). Aluminum sulfate is used to produce the blue color owners want. They should be sure to follow the instructions on the package.
NOTE: Aluminum sulfate is not recommended as an acidifier for other acid-loving plants (blueberries, azaleas, etc.) due to the potential for aluminum toxicity to these other plants. Elemental sulfur is a better choice.
Here's a great article about blue hydrangeas: http://blog.lib.umn.edu/efans/ygnews/2009/08/getting-hydrangeas-to-turn-blu.html
Here's a good article about soil pH: http://www.extension.umn.edu/distribution/horticulture/components/1731-3-soilpH.pdf
2. Too much shade. ES will tolerate some shade, but needs about 6 hours of sunlight for better blooming.
3. Pruning. ES blooms on old and new wood, so pruning the old wood removes potential flowers. A home gardener may choose to not prune the plant for a few years unless a branch becomes too long or becomes damaged / broken.
Bailey Nursery is the original breeder of ES. Here's what they have to say:
Over the last two years we have heard some reports of inconsistent flower production, especially in cooler climates. Although a number of factors may contribute to the consistency or inconsistency of blooms, there is no simple answer to this matter. Here are some steps consumers in cooler northern climates can take to help ensure beautiful blooms.
Location, location, location! Yes, that old saying is true. In Northern climates, the location of your hydrangea in the garden will have the largest impact on bloom production. The farther north you are, the more sun your plants can tolerate. In zones 4-5a we recommend planting your Hydrangeas in a location that enables them to receive at least 6 hours of sun with some dappled shade in the afternoon.
Don't treat your Endless Summer Hydrangeas like an Annabelle Hydrangea by cutting them back in the fall or early spring. By cutting to the ground or within a few inches of the ground, most if not all of the buds on old wood are being removed. In addition, the old blooms of Endless Summer add to the winter interest of your garden. Endless Summer Hydrangeas certainly do bloom on new wood, but it may take longer for flower buds to develop on the new growth of a young plant.
3. Winter Cover
Protection for plants in the first few years is important, as is protection from spring freezes. Since Hydrangea buds emerge early in spring, late freezes may damage bud development as well as any new growth. Keeping the crown of plants covered with mulch through May helps protect these buds and any soft new growth from late spring freezes.
4. Feeding your plant
Fertilization is also an important factor in flower production of Hydrangeas. A good quality, slow-release fertilizer applied once in spring or early summer should suffice for all but the most demanding locations. Look for an NPK ratio of 10-30-10. Container plants may need an additional application of liquid fertilizer during the growing season. Remember, if you over-feed your Hydrangeas, the effect is more dark green leaf production with fewer flower buds. In the North (zone 4) we recommend no fertilization after August 15th, as plants need to slow down and acclimate for winter.
The amount you water is one more factor you can regulate to ensure beautiful blooms. Although Hydrangeas are named after "Hydra", Greek for water, your hydrangeas will form large leaves, lots of green growth and few flower buds if over-watered. Over-watering may slow the formation of flowers considerably. It's normal for plants to wilt for a short time in the heat of the day. You're better off to water well and less often, than giving a little all the time.