Q • I have problem with two hydrangea shrubs. They have been planted about three years and don’t bloom. They don’t get much sun. What could be the problem? A • You don’t say what type of hydrangea you are growing. I’m going to assume the shrub we’re talking about is a bigleaf hydrangea (Hydrangea macrophylla), of which there are several hundred different varieties having either round, globe-shaped flower heads (“mopheads” or “hortensias”) or flat-topped flower heads (“lacecaps”). The bigleaf hydrangea is the hydrangea that is apt to be either pink or blue, depending on soil pH and available aluminum. Flower color may actually be white, blue, red or any tone in between depending on variety.
There are three common reasons why they may not flower: The first is winter injury. The old, flower-producing wood is frequently damaged by sub-zero temperatures. Hardiness varies with cultivar.
Bigleaf hydrangeas can also be damaged by late frosts. Most bigleafs break dormancy in late winter, and once this occurs, they become extremely vulnerable to damage from late spring frost.
Lastly, improper pruning can also prevent flowering. The right way to prune this species is to remove the stems that bore flowers immediately after flowering. Stems that did not bear flowers should be left uncut. If they don’t flower in late summer, then they have the potential to bloom next year as long as they survive winter. Stems containing dead wood can be pruned to live tissue in springtime after all chance of frost is past and the extent of cold weather injury can be ascertained. Many gardeners make the mistake of “tidying up” their bigleaf hydrangeas in fall or spring by cutting them back hard, and this is frequently why they fail to flower, even after mild winters.
To get better performance out of these marginal shrubs, consider winterizing in late fall after several hard frosts (usually after Thanksgiving). Use a soft twine to tie the stems so they are held more or less upright. Surround the stems with a cage made from wire, and wrap the outside with burlap. Fill the gap between the stems and the cage with a loose mulch such as pine needles, straw or dry leaves. The top can be left open to the weather. Remove this protection after all chance of frost is past in spring — usually sometime in mid-April.
In our climate, bigleaf hydrangeas grow well in either an eastern or northern exposure with about three to four hours of morning sun and shade during the hot afternoon hours. The planting site should have a moist, well-drained soil that has been enriched with organic matter. Of those cultivars with changeable flower color, acid soils produce blue coloring, while alkaline soils produce pink shadings.