Date: July 30, 2002

Why don't my hydrangeas flower?

Dick Bir
NC State University

First rule out the common causes of most shrubs not flowering which can often be summed up in the words "too much." Hydrangeas grow well in the shade in most of the U.S. However, there can be too much shade resulting in gorgeous leaves but no flowers. High shade from deciduous trees or pines works great in the east or from Douglas firs in the northwest. In California, any shade you can find that is not too dense seems to work and near the foggy coast or in northern areas, no shade is required at all.

Too much fertilizer, particularly high nitrogen fertilizer, will result in beautiful leaves but few, if any, flowers. Bigleaf hydrangeas can tolerate very high levels of fertilizer without showing signs of fertilizer burn so be careful and practice moderation. Too much water and soils dry can both cause a lack of flowers. I mention "too much" first because it is easy to blame the myth of flower buds forming the previous year or being on old wood and that may not be the problem. Myths are often based on truth. The buds for next year's crop of hydrangea flowers are formed this year on all cultivars of bigleaf hydrangeas we have tested. Therefore, anything that destroys or removes these flower buds from the time they are formed until they open can keep your bigleaf hydrangeas from flowering.

The two most common culprits are (1) fall, winter and spring pruning which removes flower buds and (2) cold which kills flower buds but often not leaf buds so you have a living plant with lush foliage but no flowers. This is particularly a problem in USDA hardiness zones 6 and warmer because many of these hydrangeas are listed as flower bud hardy to zone 5 or 6. The problem is that flower bud hardiness refers to dormant buds. In areas like mine with variable winter temperatures, warm late winter temperatures are often followed by temperatures cold enough to kill flower buds that are no longer dormant but starting to swell and grow.

Fortunately, hundreds of cultivars of bigleaf hydrangeas exist. Research at the University of Georgia has demonstrated that there are some cultivars that will produce new flower buds that bloom during the current season seeming to contradict the truths that we once accepted for all bigleaf hydrangeas. These are referred to as remontant bloomers. These remontant Georgia cultivars also performed well here in the cooler southern Blue Ridge mountains. They are: 'Michael Ramsey,' 'Decatur Blue,' 'Endless Summer,' 'Lilacina,' 'Mme Emile Mouillere,' 'Oak Hill' and Penny Mac.' There are also cultivars that may not be truly remontant but flower in late summer and fall. Perhaps they are enjoying the season as many humans in the upper south do during autumn. Cultivars that consistently bloomed in the summer and again in the fall near Asheville, NC were: 'All Summer Beauty,' 'Altona,' 'Blue Boy,' 'Blue Danube,' 'Blue Prince,' 'Coerulea Lace,' 'Europa,,' 'Geisha Girl,' 'Kluis Superba,' 'LaMarne,' 'Lanarth White,' 'Marechal Foch,' 'Nikko Blue,' 'Revelation,' 'Souvenier of the visit of President Doumier,' 'Veitchii,' and 'White Wave.'

If you live in an area with a variable climate or extreme cold, these cultivars would be some of my first choices for consistently flowering bigleaf hydrangeas. However, don't overlook culture. Planting on north or east slopes and under evergreen trees reduces danger from warm/cold weather cycles. Deep mulching in areas that do not have reliable cold can protect lower buds that should only be uncovered after danger of killing frost has passed. The same week you remove the mulch, you may also want to prune away the dead stems so the beauty of your lush, full bigleaf hydrangea is not impaired during peak summer flowering.

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