All About Hydrangeas


Ah, the hydrangea!  While most shade gardens can boast of at least one plant, if not more; confusion still reigns.  Questions about color, changing of bloom color, soil acidity, sun requirements and when to prune are very common and answers can be a little murky because of the sheer quantity of hydrangeas on the market and the differences between them.  Hydrangeas are starting to bloom so we thought it was a great time to have a little chat about them!

Let’s start with the most basic of basics, the different ‘kinds’ of hydrangeas.  Botanical names all start with the genus name, then the the species name, then if available, a variety or cultivated variety (a.k.a. cultivar) name.  All hydrangeas have the same genus name of ‘Hydrangea’, and there are four main species of shrub hydrangea that have been popularized and developed for our landscapes:

Hydrangea macrophylla–  These are the traditional mophead and lacecap varieties and are also known as big-leaf hydrangeas .  Mopheads have globe shaped clusters of blooms consisting of large male flowers and lacecaps have flattened bloom heads with small female flowers surrounded by larger male flowers.  The macrophylla species blooms on old wood (last years’ growth).  The only pruning necessary is removing dead stems, although additional pruning can be done before August.  Pruning in August or later could result in the removal of the following years’ blooms.  Deadheading, or removing spent blooms, can be done at any time of the year; just cut right below the spent bloom.  If cutting blooms for cut flowers, you can cut a long stem in June or July but cutting a long stem in August or after could result in reduced blooming the next summer.  These pruning tips apply to quercifolia (Oakleaf varieties) also.  Most macrophyllas have either pink or blue blooms although there are a few white ones.

Big- leaf hydrangeas are unique in that the bloom color can be changed by altering the acidity of the soil.  Blue Hydrangeas like a pH range of 4.0 to 5.0 while pink blooming hydrangeas need a 6.0 to 7.0 range to stay pink.  White blooms will stay white; no amount of working with the soil composition will change that, however, the blooms of some cultivars turn pink as they age.  Pink blooms can be changed to blue by adding aluminum to the soil and lowering pH, most commonly by adding Aluminum Sulfate (don’t over do it; plants can be killed by applying too much aluminum sulfate) or organic matter.  Since soils already have an undetermined amount of aluminum in them, bloom color can change after planting or transplanting.  Changing blue hydrangeas to pink is a little more difficult because the hydrangea has to stop absorbing aluminum.  Hydrangeas take up aluminum better as low pH levels so adding lime to raise the the soil pH can help some.  Adjusting the pH of soils in containers is easier so if you can’t make something work in the ground, consider planting it in a large container.  Remember that water has a pH too and can affect bloom color, as can foundations and walkways.

Common cultivars include: ‘Endless Summer’, ‘Glowing Embers’

‘Endless Summer’ hydrangeas are one of the most popular shade loving plants on the market, so lets spend a little more time one this one.  ‘Endless Summer’ hydrangeas differ from many macrophylla hydrangeas in that they bloom on both new and old wood; that is, branches that grew last year as well as the new branches from the present year.  Bud and blooms will continue to set throughout the season and deadheading spent blooms will encourage more flowering.

Hydrangea arborescens–  These are known as the smooth-leaf hydrangea and have big round bloom heads.  The most common arborescens cultivar is ‘Annabelle’ but more are arriving on the scene all the time.  This type of hydrangea blooms on new wood so it can be pruned during the winter with no reduction of blooms for the following season; however, do not prune in the spring as they are preparing to bloom.  Most of these have cream colored blooms but there is a new pink cultivar called ‘Invincibelle Spirit’.

Common cultivars include: ‘Annabelle’, ‘Incrediball’, Invincibelle Spirit (pink variety)

Hydrangea quercifolia– These have very different leaves than all the other hydrangeas; they are shaped like oak leafs…hence the name.  Oakleaf varieties can withstand drier soil than other cultivars but it cannot stand wet feet and will develop root rot fast in poorly drained soils.  The long, cream colored, cone-shaped blooms can be single or double depending on cultivar and turn a pink color as they age.  They also provide fall interest in the garden with leaves that turn red, yellow, orange and purple, and winter interest by displaying exfoliating bark.  Pruning of the Oakleaf hydrangeas is the same as for the macrophylla and is mentioned above.  These can take a little more sun than the macrophylla and arborescens species.

Common cultivars include: ‘Snowflake’ (double), ‘Snow Queen’, ‘Alice’, ‘Sikes Dwarf’, ‘PeeWee’

Hydrangea paniculata–  Paniculatas have blooms are in a panical shape. The growth habit of paniculatas is also much more upright than some other species and they are the only hydrangea that can be pruned into a tree.  Cream colored blooms fade into a pink color later in the season.  These can be pruned the same as arborescens since they bloom on new wood; remember not to prune in the spring or early summer as they are preparing to bloom. Paniculatas can take more sun and in cooler climates are even considered a full sun plant.  In our area, they can be in full sun if they are in rich, well-drained soil and get enough moisture, although there might still be some leaf burn.  Ideally, they would have all morning sun and some afternoon shade.

Common cultivars include: ‘Limelight’, ‘Little Lime’, ‘PeeGee’, ‘Vanilla Strawberry’

One other hydrangea species not mentioned above is the Climbing Hydrangea or Hydrangea anomala petiolaris.  This vining plant has aerial roots that can attach to materials on its own so no trellis is necessary.  The blooms are a cream color and are lacecap in form.

Encourage flowering by fertilizing with a low nitrogen, high phosphorus fertilizer (number above 30).  Stop all fertilizer applications by August 15th; this will help acclimate the plant for winter.

Adding a four inch layer of organic mulch around the base of the plant will help protect it from winter damage, although it isn’t as necessary here in Central Arkansas as in more northern climates.  If you choose to mulch the plant, do so around November 30, and then remove in late winter.  Once growth emerges in the spring, prune back old branches, about an inch above where new growth is present.

Here in Arkansas, all hydrangeas are going to do best with some afternoon shade, although as mentioned above, the paniculatas and quercifolias can take more sun.  With the exception of quercifolia, all of the hydrangeas prefer to have rich, well drained, moist soil and part-sun.  As mentioned above, the quercifolias do not like “wet feet” and do fine in drier soil.

What is part-sun anyway?  Hydrangeas and other plants that like part-sun do best planted in areas that get morning sun and afternoon shade or dappled sun all day long.

So which one is right for your landscape?  When picking out plants, take into account the mature size of the plant, bloom color, leaf texture, and sun and soil requirements.  Keep in mind that it’s better to choose the plant that works best for the site and have a healthy plant than choosing the plant that you like the best and have struggle in the chosen site.