Understanding Hydrangea

A Guide to Understanding Hydrangea

It’s the season of spectacular garden bloom.  The time of year to lounge in your lawn chair (or living room recliner – no judgment here) and enjoy the beauty of all your hard work.  Despite all the color in the garden though, the plant that stands out above the rest is the hydrangea.  The large clusters of blooms send a clear message that summer has arrived and pool parties must commence.

 

But despite the ease of which hydrangea grow, they still manage to hold a mystique over some of the gardening population.  A belief that they are difficult to grow and temperamental.

 

While this belief may be far from the truth, there are a few things you will need to know to be able to understand and successfully grow these gorgeous wonders of the garden!


What type of hydrangea do I have?

The world of hydrangea is HUGE!  We’re talking 70-75 species of hydrangea and over 600 varieties within the 6 species most common in the U.S. alone!  Let me tell you – insane family tree!!
Luckily for us, there are only 6 main species that we deal with here so I can almost guarantee that your hydrangea is one of the following:
Bigleaf Hydrangea

Endless Summer Hydrangea / Bigleaf Hydragea

Bigleaf (Hydrangea macrophylla):  hardy to zone (5) 6
Bigleaf hydrangea is also referred to as mophead and lacecap.  Leaves on this hydrangea are thick and smooth with a dark green, toothed edge.  The blooms on mophead varieties (Hydrangea macrophylla) are made up of a large cluster of showy flowers while blooms on lacecap (Hydrangea macrophylla normalis) consist of tight buds in the center surrounded by showy flowers.  While there are some white varieties of bigleaf hydrangea available, most varieties are blue, purple and pink.
Mountain (Hydrangea serrata):  hardy to zone (5) 6
While mountain hydrangea is very similar to bigleaf hydrangea, varieties tend to stay smaller in size.  They exhibit lacecap flowers that are typically blue or pink in color.

Annabelle Hydrangea / Smooth Hydrangea

Smooth (Hydrangea arborescens):  hardy to zone 4
The somewhat heart-shaped foliage of a smooth hydrangea is almost the opposite of a bigleaf hydrangea.  While the foliage is dark green with a toothed edge, it displays a matte finish and a texture that is vaguely reminiscent of a perennial instead of a shrub.  Smooth hydrangea will be covered in white flowers, though there are some pink flowering varieties on the market, from late spring into summer.
Hydrangea paniculata

Little Lime Hydrangea / Panicle Hydrangea

Panicle (Hydrangea paniculata):  hardy to zone 3
The leaves on panicle hydrangea are a medium green shade, are noticeably thinner and have a rougher texture.  Depending on variety, panicle hydrangea blooms may be cone shaped or round, full or sparse.  Sizes can range from 3 foot to 12 foot and while some blooms may fade to shades of pink and red, all panicle hydrangea bloom white.
Hydrangea quercifolia

Gatsby Gal Oakleaf Hydrangea / Oakleaf Hydrangea
Image Source: Proven Winners

Oakleaf (Hydrangea quercifolia):  hardy to zone 5
Oakleaf hydrangea is the easiest hydrangea to identify due to its leaves being shaped similar to a Red Oak tree.  All oakleaf blooms open white but often fade to pink or red.  While this hydrangea can take more shade than most of the others, it is ideally suited for partial shade to sun where the leaves will turn a lovely shade of red in the fall.
Hydrangea petiolaris

Climbing Hydrangea
Image Source: Monrovia

Climbing (Hydrangea petiolaris):  hardy to zone 4
This mid-summer bloomer is a large, woody vine that will attach itself to most surfaces.  Climbing hydrangea produces numerous white and airy blooms and prefers some afternoon protection from the sun.

When and how should I prune my hydrangea?

 Now that you’ve properly identified the species of hydrangea in your landscape you can now figure out if it’s old wood or new wood.
Old Wood Hydrangea:  Flower buds are set on last year’s growth in late summer / early fall.
(Old wood hydrangeas include Bigleaf, Oakleaf, Mountain, and Climbing.)
  • Old wood hydrangea typically like to be left alone and often don’t require pruning.  But, if pruning is necessary, to have flowers the flowering year pruning must be done in summer (yes, it will still be covered with flowers).
  • Pruning:  Remove dead stems yearly.  After the plant has been in the ground for 5 years or more, remove up to 1/3 of the stems beginning with old and weak stems.  Cut these down to the ground.
  • Removing the old blooms (deadheading):  Can be done at any time.  If you intend on cutting long stems off though, deadhead in June or July.
  • Exception:  There are some bigleaf hydrangeas that bloom on old and new wood such as the Endless Summer series.  The buds from the old wood provide early color while the new wood provides late season color.  If necessary, these can be pruned at any time.
    Old wood and new wood.

    Endless Summer Hydrangea bloom on old wood and new wood.

     

New Wood Hydrangea:  Flower buds are set on new growth in the spring.
(New wood hydrangeas include Panicle and Smooth.)
  • Pruning is seldom needed on panicle hydrangeas.  To rehab an old or floppy shrub, remove 1/3 of the old, damaged or crossing branches.  This, as well as deadheading, can be done in the late winter or early spring.
  • Because some smooth hydrangea can become floppy, they are often cut back to the ground in the winter.

 

Hydrangeas are surprisingly forgiving plants.  Pruning them at the wrong time of the year will leave you without flowers for a season but won’t permanently damage the plant.  Luckily, with the abundance of new varieties and sizes on the market every year, there is really no reason to ever have to prune a hydrangea unless you are rehabbing an old one.


Why isn’t my hydrangea blooming?

Four main issues can contribute to hydrangeas not blooming:
1.  Time:  Hydrangeas can take anywhere from 2 to 5 years before they are mature enough to bloom.
2.  Light:  Most hydrangeas need at least 4 to 6 hours of sunlight for best-blooming conditions.  Here are the ideal sunlight times for the St. Louis region:
Sunlight needed for hydrangea
3.  Pruning:  Hydrangea fall into 2 categories – those that bloom on old wood and those that bloom on new wood.
     Old Wood:  These set flower buds on last year’s growth and begin setting those buds in late summer and early fall.  They should be trimmed sparingly and, if necessary, immediately after bloom.
     New Wood:  These set flower buds on the new growth that appears in the spring.  They can safely be pruned late winter or early spring.
4.  Winter Temperatures:  It’s not uncommon for us to get cold snaps that will cause some serious dieback.  While the cold will most likely not harm panicle or smooth hdyrangea, it can do damage to bigleaf, mountain and oakleaf.
Pruning your hydrangea at the wrong time is a common reason for not having blooms.  But just because you haven’t touched your hydrangea with a pruner doesn’t mean that it’s not getting an unwanted hair-cut.  Animals, like deer, are notorious for trimming hydrangea, and while there are products that you can apply – if you live in deer haven, be prepared for some missing blooms!

Can I change the color of my hydrangea?

The flower color of white flowering hydrangea cannot be changed to any other color regardless of species or variety.  Many varieties, though, have been bred for the white flower to fade to varying shades of pink.
Occasionally the flower will fade to brown instead of the anticipated pink, but this doesn’t mean that anything is wrong with the plant.  Hydrangeas require cool evenings for the flower to change to the expected shade of pink.
Hydrangea arborescens

The color of Invicibelle Spirit Smooth Hydrangea cannot be changed.

You do have some control over the color of bloom if you have a blue/pink bigleaf or mountain hydrangea.  The color of these hydrangeas can range from pink to purple to blue and every hue in between.  These hydrangea change their color based on the amount of aluminum available to them and the pH of the soil.
Blue:  While aluminum is almost always present in the soil, it’s not always available to the plant.  The soil must be acidic to make this aluminum available for the flower color to change.  This means that the pH of the soil must be low.  Adjusting the soil to a pH of 5.5 and lower will make the aluminum available and the color will change to blue.  A pH that runs between 5.5 and 6.0 will create flowers that are shades of purple.
Soil acidifiers and Aluminum Sulfate are great ways to obtain blue flowers.  When you fertilize, use a fertilizer that is high in potassium (last number) and low in phosphorus (middle number).
True blue hydrangeas are difficult to obtain and even harder to keep in St. Louis.  Changing the color and keeping the color requires regular maintenance and attention.
One final thought on blue flowers:  If you have them planted near a house foundation, sidewalk or any other structure that may leach lime – be prepared for them to revert back to pink.
Endless Summer Hydrangea

Endless Summer flower color can be blue, purple or pink.

Pink:  To retain the pink flowers, the plant must not take up any aluminum that is present in the soil – this doesn’t seem to be a problem in the St. Louis area.  More often than not the hydrangea will either stay pink or change to shades of pink and purple in our soil.
Should you have acidic soil though, your goal pH is 6.0 and higher.  You can add dolomitic lime to help achieve this.  Use a fertilizer that is high in phosphorus (middle number) to help prevent the plant taking in the aluminum.
Whether you are adding them to a formal landscape or creating sturcture in a perennial garden, the versatility of hydrangea knows no bounds.  And with the spectacular array of varieties that are available today – hydrangea has very much become a ‘plant it and leave it’ type of shrub!

Still a bit overwhelming?  Check out  THIS  cheat sheet!!