The secret to abundant flowering: Pruning flowering shrubs

Pruning flowering shrubs is an essential practice not only to sculpt their shape but, most importantly, to ensure their vitality and stimulate generous blooms year after year. Understanding when and how to prune is key, as improper timing can compromise flower production.

Old Wood vs. New Wood: The Fundamental Distinction

The golden rule for knowing when to prune lies in understanding when the shrub forms its flower buds. We distinguish two main categories:

• Spring-Flowering Shrubs: These bloom on wood formed the previous year, known as "old wood" or "hardwood." Their flower buds develop during the preceding summer and fall.
• Summer or Fall-Flowering Shrubs: These bloom on the new shoots of the current year, called "new wood" or "softwood."

This distinction is fundamental because it determines whether spring pruning will suppress or stimulate flowering. Pruning a lilac (which blooms on old wood) in the spring before it flowers means cutting off all the promises of blooms for the year. Conversely, pruning a potentilla (which blooms on new wood) in the spring encourages the production of stems that will bear flowers a few months later.

Summer/Fall-Flowering Shrubs (New Wood): Boost Growth!

These shrubs, which colour our gardens from July until the first frosts, should be pruned in early spring, before new growth begins. The ideal period is generally in late winter or very early spring (March-April, depending on your zone and snowmelt), before the buds start to swell significantly.

Why this timing? Spring pruning removes the previous year's wood (which won't bloom or will bloom very little for these species) and encourages the plant to produce vigorous new stems. These new stems are what will bear the summer or fall flowers. This also helps control the size and shape of the shrub.

How to prune? These shrubs often tolerate more severe pruning. Start by removing all dead, weak, or damaged wood. Then, you can cut back the previous year's stems. To cut back means to significantly shorten the stems, often to just a few buds from the base (for example, cutting back to 2-3 buds for Hibiscus syriacus, or shorter for Hydrangea arborescens). For some, like Buddlejas or Hydrangea arborescens/paniculata, a shorter prune (sometimes 15-30 cm from the ground) can be done to encourage larger flowers or radically rejuvenate the plant. For hardy shrub roses, shorten the remaining stems to encourage beautiful blooms.

Common examples for our climate:

• Hydrangea paniculata (Panicle Hydrangea): Very hardy (often Zone 3). Prune in spring by cutting stems just below the dried old flower heads or shorter to encourage large flowers.
• Hydrangea arborescens ('Annabelle', 'Incrediball'): Hardy (Zone 3/4). Can be pruned severely in spring (near the ground) as it flowers profusely on new stems.
• Potentilla fruticosa (Shrubby Cinquefoil): Extremely hardy (Zone 2). Prune lightly in spring to maintain shape and remove dead wood. Can be rejuvenated by more severe pruning if it becomes leggy.
• Spiraea japonica / bumalda (Japanese Spirea, e.g., 'Goldflame', 'Goldmound'): Hardy (Zone 3/4). Prune in spring before leaves emerge to stimulate summer pink flowering.
• Buddleja davidii (Butterfly Bush): Often less hardy (Zone 5, sometimes 4 with protection); it frequently dies back to the ground. Prune severely in spring (near the ground) once the risk of frost has passed to encourage new flowering shoots.
• Hardy roses (recurrent blooming): Most modern, hardy shrub roses flower on new wood or repeatedly. The main pruning is done in spring.
• Physocarpus opulifolius (Ninebark): Very hardy (Zone 3). Although often grown for its colourful foliage, it flowers on new wood. Prune in spring for shape or rejuvenation.
• Clethra alnifolia (Summersweet): Hardy (Zone 4b). Flowering in late summer on new wood, prune in spring.

Spring-Flowering Shrubs (Old Wood): Patience Rewarded

These shrubs, which enchant us from the first warm days with their often fragrant flowers, require a different approach. It is crucial not to prune them in the spring before they flower, as this will remove all the flower buds formed the previous year.

When to prune? The golden rule is to prune immediately after flowering has finished. Depending on the species and region, this is generally between late May and mid-July.

Why this timing? Pruning right after flowering allows the shrub to dedicate the entire summer growing season to producing new stems and, especially, to forming the flower buds that will open the following spring. Waiting too long to prune (for example, in late summer or fall) reduces this crucial period of bud development and risks decreasing the following year's bloom. This synchronization ensures the continuity of the floral display year after year.

How to prune? Pruning is generally lighter than for shrubs flowering on new wood. The main goal is to maintain shape, open up the centre, and encourage the gradual renewal of stems.

Remove Spent Blooms:

This is often the first step, right after flowering. For lilacs with large flower heads (like common lilac), this improves aesthetics and prevents the plant from expending energy producing seeds. For those with smaller flower heads (Preston Lilac, Meyer Lilac), it's less necessary. For rhododendrons and azaleas, simply removing spent flowers is often sufficient. Cut just below the faded flower head, being careful not to damage the buds located just below, which are often preparing for future growth.

Thin Out and Rejuvenate:

Remove some of the oldest stems (often larger, darker, less floriferous) directly at the base of the shrub. This encourages the growth of vigorous new shoots from the base and keeps the shrub young and productive. It's often recommended to remove about a third of the old stems every 3-4 years.

Clean Up:

Remove dead, diseased, or damaged wood. Important Note: Dead, diseased, or damaged wood can be removed at any time of year as soon as it is identified, for the overall health and vigour of the plant.

Light Shaping:

Slightly shorten overly long or disorderly branches to maintain a nice shape. When pruning, always cut above a healthy bud that is facing away from the plant's centre. This encourages new growth that will grow outwards, helping to open up the shrub. Avoid severe pruning or shearing into a ball, which would sacrifice future blooms.

Common examples for our regions:

• Lilacs (Syringa spp.): Common (S. vulgaris, Zone 3), Preston (S. x prestoniae, Zone 2), Meyer (S. meyeri 'Palibin', Zone 3), Manchurian ('Miss Kim', Zone 3). Prune right after flowering.
• Forsythia: Choose hardy varieties whose flower buds withstand cold in Zones 3-5 (e.g., 'Northern Gold', 'Meadowlark', 'Northern Sun'). Prune after flowering.
• Weigela: Several hardy varieties in Zone 4/5. Prune after flowering.
• Spring Spirea (Spiraea x vanhouttei, S. arguta): Known for their cascades of white flowers. Hardy Zone 3/4. Prune after flowering.
• Magnolia: Often tree-like, they generally require little pruning, except to remove dead wood or correct young structure. If necessary, prune after flowering.
• Snowball Viburnum (Viburnum opulus 'Roseum'): Hardy Zone 3. Prune after flowering if necessary.
• Japanese Quince (Chaenomeles japonica): Hardy Zone 4/5. Prune after early flowering.
• Rhododendron and Azalea: Generally hardy in Zone 4/5 (depending on varieties). Require only minimal pruning after flowering to remove spent blooms and dead wood.

Special Case: Hydrangea macrophylla (Bigleaf Hydrangea)

This is the exception that proves the rule. Although it flowers in the summer, most traditional varieties form their flower buds on old wood in the fall. Spring pruning would therefore eliminate flowering. Furthermore, these hydrangeas are often not very hardy in Quebec (Zone 5, sometimes 4b with protection), and the bud-bearing stems frequently freeze in winter. The best approach is therefore to prune only dead wood in the spring, once the risk of frost has passed and viable buds are identified. The emergence of reblooming varieties (which flower on both old and new wood, like the 'Endless Summer' series) offers more flexibility and more assured flowering in our climate.

Shrubs with Ornamental Foliage or Stems

Some shrubs are grown primarily for their attractive foliage (often colourful or variegated) or their stems (particularly vibrant in winter). For these plants, the goal of pruning is not flowering but to stimulate the production of new shoots that will display the most intense colours.

When to prune? Pruning is generally done in early spring, before the appearance of new leaves.

Why this timing? By cutting stems severely before growth begins, you force the plant to produce an abundance of vigorous new shoots. These young shoots often have the most colourful foliage or the most vibrant stems.

How to prune? The technique varies depending on the desired effect. For vibrant winter stems (Cornus sanguinea, Cornus sericea), you can practice coppicing (cutting all stems near the ground) every year or two. For intense colourful foliage (certain Physocarpus, Sambucus), severe pruning in spring can be beneficial. For others, lighter pruning to maintain shape and remove old stems is sufficient.

Examples: Ornamental barking dogwoods (Cornus alba, C. sericea), shrubby willows (Salix integra 'Hakuro Nishiki'), Ninebarks (Physocarpus opulifolius - certain cultivars), Elderberry (Sambucus nigra 'Black Lace', S. racemosa* 'Sutherland Gold').

Summary Table: When to Prune Your Shrubs?

To help you see clearly, here is a table summarizing the pruning principles based on the type of shrub:

This table is a general guide. Always refer to the specific needs of your varieties and remember: observing your plants is the best guide for when and how to intervene. Thoughtful pruning will ensure healthy shrubs and dazzling blooms for many years.