Shrub Pruning Principles
“Foundations of good pruning lie in two primary principles. The first should be to always assess the whole plant before making any cuts, and the second is never to cut without good reason.”
(Brickell et al., 1996)
It is important when picking up the secateurs to be sure there is a good reason to prune. The most important reason to prune is to remove any dead, diseased, or damaged tissue. This will reduce the risk of pests or diseases and improve plant health and longevity. Other reasons to prune are:
To make sure the plant is safe and structurally sound
To create a nice shape that best displays the plants’ qualities
To stimulate vigorous growth
To enhance flowering and fruiting
Pruning Principles
Apical Dominance
The tallest stem grows fastest and the plant produces hormones that limit the growth of side branches. When the highest bud or branch on a plant is pruned it encourages the growth of side shoots, or bushiness.
Pruning "Hard”
In general, the harder a plant is pruned the more vigorously it will respond with regrowth. Pruning hard will not actually restrict the plant’s size, unless it is done repeatedly, because it will respond with a lot of vigorous growth. If a plant is pruned hard numerous times this can weaken the plant, so it is always best to choose a plant that fits the available space.
Pruning for Special Effects
Topiary, espallier and coppicing are all unique ways of pruning to maximize or enhance the trait of a plant. It is important to understand the plant that you are training so you can optimize your chances of it responding in a positive way.
Pruning Timing and Technique
The time you prune flowering and fruiting shrubs is critical because it will change how and where growth is induced.
Spring and early summer-flowering shrubs generally flower on the previous season’s growth and benefit from being pruned after flowering, to permit new wood to grow and ripen, then bloom in the following year. These shrubs should be pruned after flowering:
Kerria
Kolkwitzia
Osmanthus
Philadelphus
Rhododendron
Ribes
Spiraea x arguta, nipponica, thunbergii, and x vanhouttei
Syringa
Viburnum
Weigela
Abelia
Azalea
Camellia
Ceanothus
Chaenomeles
Choisya
Deutzia
Escallonia
Exochorda
Forsythia
Hydrangea macrophylla and quercifolia
Late summer- and autumn-flowering shrubs generally flower on the current season’s growth and benefit from being pruned in spring to encourage vigorous shoots that will flower later in the same year. These shrubs should be pruned in spring once growth begins:
Lavatera
Leycesteria
Potentilla
Spiraea japonica
Vitex
Buddleja
Callicarpa
Caryopteris
Fuchsia
Hibiscus
Hydrangea paniculata
When flowers are borne on spurs arising from a framework of old wood; the spur wood is pruned back to the framework at the appropriate season, depending on flowering time. Includes:
Wisteria
Grape
Kiwi
Apple
Some plants do not fall into an easy pruning category because it depends how you would like to utilize its ornamental attributes.
Berberis: Responds well to renovation, but can be clipped with shears. Looks more attractive if pruned selectively and thinned.
Buddleja globosa: Flowers are produced on shoots from the previous season’s wood so prune only as needed. If needing a renovation, do this in early spring.
Boxwood: Prune to shape in April to May when on a dry day. Pruning may need to be done again in September.
Cistus: Young plants should be pruned in spring to encourage bushiness. Old plants do not respond well to pruning.
Cornus sericea syn stolonifera: Hard annual pruning in early spring produces the best winter stem colour, but at the expense of flowers
Cotinus: To grow as a flowering shrub keep pruning to a minimum. For larger leaves, prune hard in spring.
Erica/Calluna: Prune as soon as flowers fade and new growth appears
Hebe: Shorten stems after flowering to keep the habit compact. Can tolerate hard cutting back to remove winter damage.
Pyracantha: Flowers are borne on spurs from the old wood. Espaliered shrubs can be pruned in late summer, shortening growth to expose the berries. Prune hedges in the spring and clip two to three times during the growing season.
Sambucus: Cut down half the stems to ground level and shorten the remainder by half. More severe pruning can be used to enhance the foliage effect.
When plants “bleed” when they are cut, for example Japanese Maple, they should not be pruned in late winter, when the pressure of rising sap will cause profuse bleeding. Also some broadleaf evergreens, if pruned too early in spring or too late in summer, will produce soft new growth that may be damaged by frost or cold.
Pruning Clematis
How to prune clematis is dependent on what “group” they are in. The clematis group is determined by the vines season of flowering and the age of wood that bears flowers.
Group 1: Generally, single flowers appear in later spring to early summer on the previous season’s ripened growth. Also know as group A.
When to prune: Immediately after flowering (if required)
Training and pruning: Established plants only need pruning to train to size. Cut back shoots that are too long. Remove winter damaged growth after last frost. Old and congested plants can be thinned
Types: C. alpina, C. armandii, C. montana, C. paniculata
Group 2: Typically, large-flowered hybrids that have two flushes of flowers, first around May or early summer on previous year’s wood, and then again in September or late summer on new shoots. Also known as group B.
When to prune: Late winter or early spring before new growth begins
Training and pruning: Goal is to retain a framework of also wood, while also stimulating new shoots to maximize flowering. Stagger pruning by cutting back some shoots to healthy buds later to others. This will prolong flowering. Minimal pruning is an option, and simply cut back hard every three or four years. In the first year the first flush of flowers will be reduced.
Types: ‘Asao’, ‘Barbara Dibley’, ‘Barbara Jackman’, ‘Bees’ Jubilee’, ‘Countess of Lovelace’ ‘Daniel Deronda’, ‘Fair Rosamond’, ‘H.F. Young’, ‘Horn of Plenty’, ‘Jackmanii Alba’, ‘Jackmanii Rubra’, ‘Nelly Moser’, ‘Niobe’, ‘Princess of Wales’, ‘Proteus’, ‘Snow Queen’, ‘Vyvyan Pennell’
Group 3: Single flowers or large flowers appear in later summer on growth made in that season. Also known as group C.
When to prune: Late winter or early spring when buds show signs of growth.
Training and pruning: Established plants should be prune hard every year or vigor and flowering will reduce overtime. Cut stems to strong pairs of buds 12 inches above ground level. Remove dead stems.
Types: C. integrifolia, C. tangutica, C. viticella
Brickell, C., & Joyce, D. (1996). The American Horticultural Society pruning and training (pp. 13–15). essay, DK Pub.