Q. I'm having problems getting hydrangeas to bloom. I have three 'Nikko Blue' that don't bloom at all. I also have three traditional white hydrangeas. One of them, the one that wasn't moved, blooms. But the two plants that were moved three years ago don't bloom. How can I get all my hydrangeas to bloom?
A. You have two different species of hydrangeas. Each has a different growth and blooming pattern.
'Nikko Blue' is a popular cultivar of bigleaf hydrangea (Hydrangea macrophylla) that is grown for its large, mop head-type clusters of blue flowers. This species produces flowers primarily on old wood, which means that any flowers that bloom this year began as tiny buds set on the stems of last year's growth.
Unfortunately, the stems of bigleaf hydrangea are not hardy in our area (Zone 4). Bigleaf hydrangea is considered stem hardy only to Zone 5 at best. In Minnesota, the stems are almost guaranteed to be killed back to the snowline or to the ground each winter. Without snow cover, the entire plant is likely to be killed by our sub-zero temperatures. If the stems are killed over winter, so are the flower buds that would have produced flowers in the summer.
Some Zone 4 gardeners manage to get bigleaf hydrangeas to bloom by providing extensive winter protection to keep the stems from freezing. You would have to do this to get reliable bloom on your 'Nikko Blue' hydrangeas.
There are some bigleaf hydrangeas that also produce a few blooms on new wood later in the summer. So even if the main blooms are killed by winter die-back, there may be a few blooms on the current year's growth. This trait of having some bloom on new growth seems to vary by cultivar and a recently introduced cultivar, named Endless Summer, was selected because it reliably produces blooms on new wood. Even though the stems of this cultivar will die back in Zone 4, it should provide a nice floral display for gardeners in this region.
I'm guessing that your other hydrangeas are smooth hydrangea (Hydrangea arborescens), a shrub that produces many upright stems 3 to 4 feet tall and blooms in mid to late summer. The most commonly grown are the snowball-type cultivars such as 'Annabelle' or 'Grandiflora' (which is also known as 'hills of snow hydrangea'). These cultivars bear very large, rounded clusters of white flowers.
Smooth hydrangea's flowers are produced on the current year's growth. Even if last year's stems died back or were pruned back, the new stems that develop this year will bear flowers. Winter dieback is rarely a problem, since this species is hardy to Zone 3.
Since the smooth hydrangea that was not moved is blooming fine, I suspect that the lack of bloom on the two that were moved is related to transplant shock. Digging up a shrub, no matter how carefully, removes a large portion of the plant's root system. It may take several years for the plant to re-establish in its new location. The plant will direct its energy toward a root system rather than flowers until it is well established.
Flowering could also be delayed or reduced if you moved the plants to a heavily shaded spot. To encourage your smooth hydrangeas to bloom, water as needed, apply a balanced fertilizer once a year (avoid excessive nitrogen, which can hinder bloom), and you should have a blooming shrub soon.
Nancy Rose is a horticulturist, writer and photographer. To ask her a gardening question, call 612-673-9073 and leave a message. She will answer questions in this column only.