Why Timing Matters More Than Technique

Most spring flowering shrubs make their flower buds in the summer before they open. Forsythia, philadelphus and flowering currant all set next year's blooms on stems grown this season, which is why a tidy-up in autumn or a hard cut in February does so much damage. You are not pruning badly — you are simply removing the flowers before they have had a chance to appear.

The rule that saves you from that mistake is straightforward: prune immediately after flowering, and aim to finish by mid-summer. That gives the shrub the rest of the growing season to push out new shoots, ripen them and set buds for next spring. A forsythia trimmed in late May looks a little sulky for a fortnight, then fills out and rewards you the following March. The same shrub cut in November will simply sit there, flowerless, while you wonder what went wrong.

Know What You Are Cutting

Before you pick up the secateurs, work out which group your shrub belongs to. It takes two minutes and prevents a wasted year.

• Flowers on old wood — prune straight after flowering: forsythia, flowering currant (Ribes), philadelphus, deutzia, weigela, kerria, exochorda, chaenomeles, lilac, magnolia and most rhododendrons and azaleas.
• Flowers on new wood — prune in late winter or early spring: buddleja, Hydrangea paniculata, caryopteris, hardy fuchsia and perovskia.

If you are genuinely unsure, leave the shrub alone for a season and watch when it flowers. A year of slightly untidy growth is a small price for getting the timing right.

The Fortnight That Matters

For the old-wood group, the window is short. Start deadheading as the petals drop — snapping or snipping spent blooms off rhododendrons, lilacs and magnolias as you go — then carry out the structural pruning within about two weeks of the last flower fading.

Do not drift into July with early shrubs. Forsythia, for instance, finishes flowering in April, so a cut in late June leaves new growth with barely enough time to harden and form buds before autumn. The later you prune, the softer and more sappy the regrowth, and the less likely it is to flower next year.

What to Cut — and What to Leave

Once the flowers are gone, tackle the shrub in three passes, working from the obvious problems inward.

• Dead, damaged and diseased wood. Cut back to clean, healthy wood or right down to the base. Anything rubbing or crossing should go too, because wounds there invite disease.
• Crowded stems. Take out the oldest, thickest stems at ground level to open up the centre of the shrub. Air and light reaching the middle means better ripening, fewer fungal problems and flowers further in rather than only at the tips.
• Flowered shoots. Shorten these back to a strong pair of buds or to a sideshoot lower down. This is what keeps the shrub bushy instead of leggy and bare at the base.

Make every cut cleanly, just above a bud that faces outwards, sloping away from it so water runs off. As a general guide, remove no more than a quarter to a third of the oldest stems in any one year. With philadelphus, deutzia and weigela, taking out the two or three oldest stems annually is usually enough to keep them young. Lilac is a special case: deadhead it and remove suckers, but avoid cutting into thick old wood, because lilac is slow and often reluctant to regenerate from it.

Renovating an Overgrown Shrub

If a shrub has been neglected for years, resist the temptation to cut it to the ground in one go. Spread the work over two or three years, removing about a third of the old stems each spring immediately after flowering. The shrub keeps its shape, keeps flowering and never suffers the shock of total defoliation.

Some plants take hard renovation well. Kerria, philadelphus, deutzia and weigela will usually shoot strongly from the base if cut back severely, and they recover quickly. Others will not: lilac, magnolia and most rhododendrons resent being cut into bare, old wood, and may never break from it. With those, work gradually, keep the pruning light and accept that a full recovery may take several seasons.

Aftercare and a Few Cautions

Pruning is a shock, so give the shrub a good start afterwards. Scatter a balanced general fertiliser around the root zone, then mulch with a generous layer of well-rotted organic matter, keeping it clear of the stems. Water thoroughly in dry spells through the first summer, especially after renovation work, so the new shoots develop properly rather than sitting still.

• Use sharp secateurs and loppers — blunt blades crush stems and leave ragged wounds.
• Clean blades between shrubs with methylated spirit or a diluted bleach solution, particularly after cutting anything diseased.
• Never prune when the wood is frozen or the stems are wet; both encourage infection.
• Leave magnolias until mid-summer if you must cut them, as they bleed badly in late winter and spring.
• Do not cut rhododendrons back into bare wood with no visible growth buds — they rarely shoot from it.

Get the timing right, thin rather than chop, and feed afterwards, and your spring shrubs will keep flowering generously for decades with very little fuss.