THREE COMMON REASONS MOPHEAD HYDRANGEAS FAIL TO BLOOM: A late spring freeze arrives and ruins the developing bloom buds. The freeze may be light and even go unnoticed until one realizes that the blooms are not forming. Or it may kill all the emerging leaves, too. As a result, most of the new growth comes from the roots (as in the picture above). When this occurs, you know you have a problem. Most flower buds develop on the old stems. Once these stems are damaged in a late freeze, new flowers will not appear until the following year and only then if it is a milder spring. (This is the rule for the vast majority of mopheads [macrophyllas] but there are exceptional hydrangeas that will bloom despite this damage) Improper pruning.
-PRUNING MADE EASY (Hopefully) FIRST, I want to make clear that hydrangeas do not usually need to be pruned - ever - except to remove dead stems and blooms. But if your hydrangea is getting much too large, use one of the following methods to prune it.
PRUNING is divided into two methods. Use Method One if you have mophead or lacecap hydrangeas (these are usually blue or pink) or if you have Oakleaf hydrangeas (leaves shaped like large oak leaves, white blooms). Use Method Two if you have paniculatas (PeeGees) or Annabelle (both of these bloom white). Prune these hydrangeas only in the summer BEFORE August (to be safe). Some experts believe these hydrangeas may be pruned even into August, but this might be risky. The hydrangeas may already have set their bloom buds for the next year (especially if they are growing in Zones 5 and 6).] Method I pruning is for hydrangea types that bloom on OLD WOOD. (Stems are called "old wood" if they have been on the hydrangea at least 9-10 months before the hydrangea blooms.) This means that flowers are formed on the stems of hydrangeas around August, September or October for the following spring. If those stems are removed (pruned) in the fall, winter, or spring, the bloom buds will be removed and there can be little or no bloom that year. Occasionally a mophead hydrangea will defy this general rule and bloom after being pruned at the "wrong" time. But for the vast majority of hydrangeas, pruning after about July/August will eliminate the bloom for the next spring. This does not mean that hydrangeas can never be pruned. All dead wood should be removed from hydrangeas every year. After the plants are at least 4 years old, about 1/3 of the older stems can be removed down to the ground each summer. This will revitalize the plant. In addition, if it becomes necessary to prune a plant to reduce its size (temporarily), it may be cut back in June or July without harming the next year's bloom.
REMOVING OLD BLOOMS Removing old blooms on a plant is called "Deadheading." Fortunately, we can remove the old blooms at any time of the year without harming the bloom for the following year. In June and July you may remove them in any way you would like (long stems or short stems). Here are some tips for deadheading: (1) When you remove blooms in June or July, you can cut them with long stems because the bloom buds haven’t set for the following year. (2) When you remove them after the first of August, it would be safest to remove them with very short stems so you won’t disturb any developing bloom buds for next year. As long as you cut above the first set of large leaves, you will be fine. PRUNING: METHOD TWO For paniculata and 'Annabelle' Method II is for hydrangea types that bloom on new wood. It is a joy to grow these type hydrangeas because they are determined to bloom every single year, no matter how they are pruned. The only time they cannot be pruned is immediately before they are to bloom. Many people grow hedges of Annabelle and cut them within a few inches of the ground each fall so they will not be an eyesore during the winter. They will still bloom beautifully in the spring/summer. Go to the page on 'Annabelle' for a more detailed description of the pros and cons of pruning this hydrangea to the ground. While at this page, view a picture of Penny McHenry's Annabelle hedge. Paniculatas can also be pruned in the fall or winter. After pruning, they will grow 5-7 feet in the spring and summer and then put on a beautiful show. If one is attempting to grow a paniculata in a tree form, however, you would not want to cut off the developing trunk or main branches. I suggest trimming out crossing branches and those that do not contribute to an attractive form. Planted in wrong zone. If you have had the bad luck to plant a hydrangea that has never bloomed after the first year, you may finally have to concede that this particular variety is not cold hardy in your area. Another variety may succeed famously, or you may have no luck with mophead hydrangeas. If your hydrangea has never bloomed, don't give up hope. See What Type Hydrangeas Can I Grow? I have found that often the hydrangea that is not cold hardy was received as a gift or bought wrapped in foil. This is a sign that it is directly out of a greenhouse and may not be adapted to your location. Some "gift type" hydrangeas are perfectly hardy, depending on where you live. But one will be much more successful in growing hydrangeas if he/she will purchase them from a local nursery. Local nurseries try to stock hydrangeas that are known to do well in the area. It also helps to ask the nurseryman (it pays to shop where there is an expert!) if this particular hydrangea is temperamental to cold weather. advice courtesy http://www.hydrangeashydrangeas.com/ great website covering the world of hydrangeas