Pruning Shrubs, Part 4 - A Plant by Plant Guide

Here, we’ll put the principles of the why, when, and how to prune in context for specific shrubs. Remember, it is not necessary to prune plants every year – in fact, we select our shrubs specifically to be low-maintenance and easy to grow. Before you put the instructions below into action, go back and read part one of our pruning series to determine if you need to prune in the first place. When in doubt, don’t prune!

Hydrangeas

We’ll start with hydrangeas since they are the most often asked about shrub when it comes to pruning. Three main types of hydrangeas are commonly found in our gardens; none of them require annual pruning to perform well. However, if you’ve decided that your hydrangeas need pruning, it is important to know which type you have to make the right decisions about when and how to get the job done.

Bigleaf hydrangea, known botanically as Hydrangea macrophylla, is sometimes also called mophead hydrangea, lacecap hydrangea, florist’s hydrangea, or hortensia. It is a popular gift plant as well as a landscape plant, with big, round flowers in pink, purple, or blue.  Bigleaf hydrangea is the plant that throws a wrench into all of the conventional wisdom and advice about pruning, because it flowers in late summer on old wood. It is the only popular landscape plant to do this, which makes it especially tricky to prune.

The best advice on pruning big-leaf hydrangea is to avoid doing so if at all possible, because you cannot prune it without sacrificing some flowers. If you prune before it blooms, you remove flower buds. If you prune after it blooms, it won’t have enough time to grow the mature wood that is capable of producing flower buds for the following year. Even in the case of reblooming hydrangeas like the Let’s Dance series, which flower on both old and new wood, avoiding pruning ensures the longest display and showiest performance. We select all of our big-leaf hydrangeas for a nice, rounded, compact form, so they don’t require pruning to look handsome and tidy, though any remaining flowers from the previous year should be removed when new growth begins in the spring. If you’ve inherited an older type of hydrangea, or moved to a home with a large, unruly hydrangea, the best management strategy is to shorten one-third of the stems every year after flowering. In this way, two-thirds of the plant remains to flower every summer and the plant can be maintained at a lower height.

Oak leaf hydrangeas and climbing hydrangeas, which are not as widely grown as big leaf hydrangeas, also flower on old wood and shouldn’t be pruned if possible. If, however, they must be, follow the guidelines above for big-leaf hydrangea.

The other two popular types of hydrangeas – smooth hydrangea (Hydrangea arborescens – sometimes called ‘Annabelle’ hydrangea) and panicle hydrangea (Hydrangea paniculata – also known as peegee hydrangea) – flower on new wood, so can be pruned and still flower that same summer. It’s a good idea to go out in early spring and remove any skeletons from last year’s blooms that are still on the plant, as these may not fall off naturally and can spoil the look of this year’s flowers. Aside from that recommendation, these plants do not strictly require regular pruning, though if do you do want to prune them, they will still flower if that summer. One-third of their height may be removed each spring to encourage new growth and branching if desired.

The following plants may be pruned in early spring, just as the new growth begins to emerge on the stems:

Abelia
Butterfly Bush (Buddleia)
Beauty Berry (Callicarpa)
Bluebeard (Caryopteris)
Summersweet (Clethra)
Burning Bush (Euonymus)
Rose of Sharon (Hibiscus)
St. John’s wort (Hypericum)
Goji berry (Lycium)
Potentilla
Rose  

The following plants may be pruned later in the season, immediately after they finish flowering:

Carolina allspice (Calycanthus)
Quince (Chaenomeles)
Red twig dogwood (Cornus)
Deutzia
Pearlbush (Exochorda)
Forsythia
Sweetspire (Itea)
Beautybush (Kolkwitzia)
Ninebark (Physocarpus)
Rhododendrons and azaleas
Elderberry (Sambucus) *note – if growing for fruit, elderberry cannot be pruned without sacrificing flowers/berries
Spiraea
Lilac (Syringa)
Weigela 

Shrubs that flower on old wood and offer ornamental fruit cannot be pruned without sacrificing their display, so it is best to avoid pruning these all together:

Winterberry holly (Ilex verticillata)
Blue holly (Ilex x meservae)
Viburnums  - if grown for the berry display – varieties grown only for their flowers, like Koreanspice viburnum, may be pruned immediately after flowering if necessary
Coralberry (Symphoricarpos)

Conifers and evergreens that are not grown for their flowers or fruit should generally be pruned in late spring or early summer, after new growth has already begun but before temperatures get too warm.

This group includes:

Boxwood
Wintercreeper (Euonymus)
Bay laurus
Japanese holly
False cypress (Chamaecyparis)
Juniper
Arborvitae (Thuja)
Russian cypress (Microbiota)

Remember – when in doubt, don’t prune! It’s easier to correct an unpruned shrub than an incorrectly pruned shrub. If you still have questions, we’re always here to help. Contact us any time under questions and feedback on the left hand side.