Hydrangeas- Color, Pruning, Culture and Some Favorites (Part 2)

Last week I covered the major species of Hydrangeas, flower form, culture and problems here in New England (part 1).  This week, I'll cover managing color, pruning, care and some favorites.


Changing color of hydrangea flowers

macrophylla Endless Summer @ Millican Nurseries

This is really an issue for the mophead and lace cap varieties of the macrophylla/serrata species which includes the Endless Summer varieties (not Bella Anna which is arborescens).  Some of the common and well known of this species are: Nikko Blue, the Cityline series, Toyko Delight, Lady in Red, Pia, Penny Mac, Variegata and many more.  If you prefer a color, choose a variety that is bred for the color you like, it will be easier to keep it in the range.  Remember the flowering challenges for these plants in zones 5 and 6, with the exception of the Endless Summer series.

The major controlling factor in the flower color is the soil pH.  If you like pink flowers, then an alkaline soil (pH >7.0) is required, and if you want blue flowers you need acid soil with a pH well below 6.0.  Sulphur is an acidifier while lime makes the soil more alkaline.  Check out a previous post, It's all about the Soil for more details on managing pH.

Nutrient availability with changing
pH levels from UMass

However, it is Aluminum from the soil that actually makes the flowers blue, and Aluminum is only available to be absorbed by the plant with a low soil pH.  The chart shows how pH levels effect a plant's ability to absorb nutrients, and Aluminum and Iron have the same characteristics and are only available to plants in acid soil.

You can always try adding nutrients blindly, but I always suggest a soil test so you can precisely understand your soil conditions.  To ensure pink blooms, you want to make sure the soil is alkaline which inhibits the plants ability to absorb Aluminum.  The UMass soil test states levels of extractable Aluminum, and if you have sufficient levels, Sulphur is all that is needed to acidify your soil.  Sometimes there may not be enough Aluminum and you need to add Aluminum Sulfate, which adds both Aluminum and Sulphur.  I prefer not to add metals to my soil unless they are needed.

paniculata 'Pink Diamond' @ Millican Nurseries

As another common example of how pH effects a plants ability to absorb nutrients, Rhododendrons and Azaleas like acid soil, which is why they perform so well in New England's naturally acidic soil.  When the soil is more alkaline than they like (pH >6.5), their leaves will often turn chlorotic (yellowish), and this is the result of the plants not being able to absorb enough iron at the higher, alkaline pH levels.  Rhododendrons and Azaleas need iron to keep their leaves lush and green, and it is mostly available in acid soil, and when their leaves turn yellowish, it is often (not always) an indicator that the soil pH is to high.

So, while pH is the critical factor, it is not the cause of the change in hydrangea flower color.  Do realize that changes in soil chemistry can take months or a season to take effect, and the soil often will revert to its natural levels without constant management.  Also realize that a beautiful blue hydrangea that you plant may well be pink the next year if you don't know the soil conditions you are planting in.   You need a little time, patience and science to achieve the color want, or you can just plant them and let them do their thing!

quercifolia 'Snow Queen' @ Millican Nurseries

How and when to Prune

The best way to minimize pruning any plant is to purchase the right plant for the right location.  With all the varieties of Hydrangea nowadays, it should be easy to find the right sized plant to minimize pruning.

For the macrophylla species, that flowers on old wood, I find it is best to prune right after flowering as the flowers begin to fade, and prune back to a strong branch with buds.  This ensures the plant has time to put energy into the wood for next year's flower development.  Prune back far enough into the plant so it has space to grow and only make a few major cuts each season.  Each year it is also good to remove a couple of older, woody stems to the ground, which keeps the plants open and allowing new growth to generate from the base.

paniculata 'Limelight' @ Millican Nurseries

The idea is to do a little each year and never past mid-summer or in the spring.  Improper pruning by contractors is the major reason for poor hydrangea flowering.  It is the major reason for many plants not flowering, but that is an issue for another day.  The only pruning in the spring is to cut out dead wood, usually done after the plants leaf out so you can be sure.  Ideally, your hydrangea is in a place where it requires no pruning, except the occasional nip and cutting out of dead stems.

The exception here are the Endless Summer varieties (Endless Summer, Blushing Bride and Twist 'n Shout)  These newer plants flower on old and new growth.  I find their structure to be less pleasing than the traditional macrophylla species, but because they flower throughout the season, I think they benefit from occasional nipping to clean-up a somewhat messy form.

macrophylla 'Blushing Bride' @ Millican Nurseries

For the quercifolia or Oakleaf Hydrangea, the timing is just like the macrophylla.  It flowers on old wood and should only be pruned just after flowering.  This plant tends to be much bigger and is structured more like a shrub as opposed to having stems coming from the base like the macrophylla.  If any pruning is necessary for shaping, prune back to a bud on a strong branch.

The arborescens and paniculata both flower on new wood, so they can be pruned after flowering through the end of the season.

The arborescens has a stem form like the macrophylla, and your can prune this to 6" inches of the ground each fall.  This will keep the plant smaller, but the stems are weaker and don't have the strength to hold the wet flower.  Or, you can trim it back in the fall to buds on a strong set of stems.

arborescens 'Invinceball' ©BDG

The paniculata can become a big nasty rats nest of branches and twigs.  If you want to keep the flower heads on for the fall and winter, you can prune in the spring.  Depending on how much space and where these hydrangeas are sited, determines how much work you want to do.  I like to do a little work each year on trying to keep the shrub open and clean up the crossing network of twigs and branches.  This allows the plant to hold the flowers better and look a little nicer over the winter.  Or, you can just let them go and prune out the random branch that gets out of control.  You can prune these in the spring and still get excellent flowering in the late summer because they flower on the season's new wood.

For all Hydrangeas, sometimes you just want to start over.  You can cut them down to any framework or size, just realize you may lose a season of flowers.

Proper feeding and culture

The key to lots of flowers on your hydrangeas, is to not pamper them to much.  They need good soil, so scratch in compost every year.  This is the single best  practice for a plant like Hydrangea.  The good soil with compost and mulch on top will hold water and keep the plant's roots moist.  I add triple super phosphate instead of a balanced fertilizer because too much nitrogen will give you a lush green plant and few flowers.  The compost will provide the other nutrients.  Phosphorous is the key nutrient for producing flowers, and I find it a great addition for boosting flowering shrubs.

paniculata 'Quickfire' @Millican Nurseries

For the macrophyllas, they require good sunlight, an eastern exposure ideally gives them morning sun and protection from the wilting afternoon sun.   Plants that are constantly wilting will not produce well. The arborescens can't take the full sun either and produces well in the shade where the macrophyllas do not.  The paniculata can take the most neglect, and they will perform well in average soil and part shade to full sun.

Some Favorite Plants 
(most of these link to Millican Nurseries, a great wholesale nursery I use in New Hampshire)

Hydrangea macrophylla 'Endless Summer'
May not be the best looking shrub, but with good care loaded with flowers all summer long in zone 5 and 6.

Hydrangea macrophylla 'Blushing Bride'
Same as Endless Summer but a white flower fading to blush.

Hydrangea macrophylla 'Twist 'n Shout'
Best flowering of the Endless Summer series, is a lace cap but flowers like crazy.

Hydrangea macrophylla 'Lady in Red'
Intriguing shrub beyond just flowers but tough in my zone 6. I would love to grow this on the Cape

Hydrangea paniculata 'Limelight' and 'Little Lime'
Great paniculata that emerges lime colored and then fades to white and on.

Hydrangea paniculata 'Pink Diamond'
Pink flowers that age to bright red.

Hydrangea paniculata 'Quickfire'
Emerges white and ages to bright red.  Flowers a month earlier in summer than other paniculatas.

Hydrangea quercifolia 'Snow Queen'
Beautiful and floriferous Oak-Leaf cultivar