The Rules of Hydrangeas

Posted 7/14/19

Have you ever heard the old saying “for every rule there is an exception”? We as home gardeners love it when our plants “follow the rules” so to speak. Plant at a certain time, fertilize at a …

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The Rules of Hydrangeas

Posted

Have you ever heard the old saying “for every rule there is an exception”? We as home gardeners love it when our plants “follow the rules” so to speak. Plant at a certain time, fertilize at a certain time and in a certain way, prune at a certain time, etc. However, not all plants tend to always follow the rules. One such rule (and one that I have shared with many folks over the years) is the May 1st rule. If a plant blooms prior to May 1st it should be pruned immediately after flowering (azaleas and yellow bells for example) and if it blooms after May 1st then prune it during the winter dormant season (crape myrtles for example).

Among my personal favorite plants are the hydrangeas. Who can’t remember the large spreading “mophead” hydrangeas that bloomed each summer outside Mother or Grandmother’s house. Most people don’t realize, however, that there are actually many different types or categories of hydrangeas not all of which follow the May 1st rule when it comes to pruning, so before pruning your hydrangeas it is very important to know which kind of hydrangea that you actually have.

Some plants (hydrangeas included) flower on “old wood,” meaning blooms are set on stems produced from last year’s buds. These plants are usually the ones blooming during early spring. Since these plants bloom before May 1, prune immediately after blooms fade or finish.  Other plants set buds on this year’s growth, also called “new wood.” If flowering occurs after May 1, prune plants during the dormant season, in late winter or early spring. Following this general rule with hydrangeas could lead to a bloomless plant and an unhappy gardener.

One of the most familiar and certainly most common of all the hydrangeas is hydrangea macrophyllia or what we commonly call the “mophead hydrangeas”. This is the type of hydrangea that most of us remember from grandmother’s house. One interesting tidbit about most of these hydrangeas is that their bloom color can change depending upon the pH of the soil that they are growing in. In low pH soils they take on a blue color due to the fact that the element aluminum (which gives the flower its blue color) in the soil is taken up by the hydrangea. In relatively high pH soils aluminum cannot be taken up by the plant resulting in a more pinkish bloom color. We can actually control the bloom color to some extent in these type hydrangeas. If you want pink blooms apply lime to raise the pH and if you prefer blue then you can apply sulfur or fertilize with a high acid fertilizer such as aluminum sulfate. Not all of them respond in this manner as some newer mophead type hydrangeas bloom true to variety regardless of soil pH. Mophead hydrangeas typically bloom early- to mid-summer. If gardeners followed the May Rule according to timing, these hydrangeas would fall into the dormant season pruning category. However, that is not the case. Mophead hydrangeas bloom only on old wood and should be pruned soon after flowering.  One of the most common calls I get about hydrangeas is that they fail to bloom year after year. The two most common causes of this is late spring freeze damage (remember that they bloom on old wood and once the bloom buds have been destroyed you will miss a season of blooms) and the other results from pruning this type of hydrangea during the dormant season which removes their bloom buds. Prune them right after their first large flush of blooms have faded.

Another category of hydrangeas is the native oak leaf hydrangea that can often be seen along the edges of the woods. Oak leaf hydrangeas have also gained a lot of popularity in home landscapes and native areas around your home. There are many named cultivar varieties now ranging from dwarf two foot plants up to large shrub size and we even have color variations now as well. They are great plants for those native plant lovers out there, and in fact oakleaf hydrangea is our official state wildflower. This group also includes the “Snowflake” oakleaf hydrangea that was actually developed at Aldridge Gardens down the road in Bessemer. Oakleaf hydrangeas also bloom on old wood so they likewise need to be pruned immediately after flowering. I usually recommend completing your pruning on both oakleaf hydrangea as well as your macrophyllia (mopheads) by late July at the latest as they will begin to set next year’s bloom buds in late July or early August.

The final group of hydrangeas is one that I think we vastly underuse in our landscapes. For whatever reason, they just seem to get forgotten about.  This group is our pee gee or panicle hydrangeas. Pee Gee hydrangeas are certainly not new as they were actually a garden staple way back as far as the 1800’s. Like with other plants, we now have many newer varieties or cultivars which vary greatly in size, form, and color. In my mind the pee gee type hydrangeas have two great advantages. First of all they take more full sun than our other hydrangeas. It would still be preferable to protect them from the west sun; however, they will do fine from full sun to partial shade unlike the “mopheads” which prefer partial shade and suffer greatly in full sun. Secondly, they have the great advantage of blooming on new wood which means they are more cold tolerant (if a late freeze damages the bloom buds they will put on new growth and still bloom just fine because their bloom buds are on new wood). This also means that we prune these hydrangeas differently. Pee gee hydrangeas should be pruned in the winter dormant season or even in the very early spring.