Gadenin日本語Get the appWhy shearing costs you the flowers
Viburnum sets its flower buds on wood that has already had a full season to ripen. By the end of summer, the buds for the following display are formed and sitting at the tips and upper joints of that older wood, waiting out the winter. This is what people mean when they say viburnum flowers on old wood, and it has one blunt consequence: whatever you cut off the outside of the bush is exactly where the flowers were going to be.
A shrub trimmed with shears once or twice a year will stay tidy, thicken into a dense shell of twiggy growth, and flower less every season until it barely flowers at all. The owner usually blames the soil or the site. The cause is the shears. The tip growth being removed is the flowering wood, and the dense outer shell that shearing produces also shades out the interior, so the plant has nowhere else to make replacement wood.
The alternative takes less time, not more. Instead of trimming the surface, reach into the plant and remove whole stems at or near ground level, choosing the oldest and the most crowded. The shrub keeps its natural outline, light reaches the middle, new stems come up from the base to replace what you took, and the flowering wood on every stem you left alone is untouched.
Deciduous and evergreen types are pruned at different times
The two broad groups want opposite timing, and getting this the wrong way round is the second most common mistake after shearing. Deciduous viburnums that flower in late winter or spring are pruned immediately after the flowers fade, which gives the whole growing season for replacement wood to grow and ripen its buds. Deciduous types grown mainly for autumn berries are pruned lightly, and only after the birds have taken the fruit, because pruning after flowering removes the developing berries.
Evergreen viburnums are pruned in late spring, once flowering has finished and the danger of hard frost has passed. Cutting an evergreen in autumn or winter exposes soft inner growth and fresh cut surfaces to cold at the point when the plant cannot heal them, and the result is browning and dieback along the cuts. Late spring pruning is followed immediately by warm weather and a flush of new growth that covers the work within weeks.
The winter-flowering deciduous types sit slightly apart. They open a few flowers at a time across the coldest months, and they are best left alone until the last of them has gone over in early spring. Prune those the same way as the spring-flowering group, just later on the calendar than you might expect.
| Type | When to prune | How much to take | Reason for the timing |
|---|---|---|---|
| Deciduous, spring-flowering | Straight after the flowers fade | Up to a quarter of the oldest stems | Maximum time for new wood to ripen buds |
| Deciduous, winter-flowering | Early spring, as the last flowers go over | Light thinning only | Flowering runs on for months, so wait for the end |
| Deciduous, grown for berries | Late winter, after fruit has been eaten | One or two old stems only | Pruning after flowering cuts off the developing fruit |
| Evergreen | Late spring, after flowering and after hard frost | Reshaping cuts, plus one old stem | New growth follows at once and covers the cuts |
| Any type, overgrown and bare at the base | Over three successive years, in its own window | A third of the oldest stems each year | Keeps some flowering wood while the plant rebuilds |
Renewal pruning, stem by stem
The method is the same for every type; only the season changes. Work from the base with a pruning saw or loppers rather than from the outside with shears, and take out complete stems rather than shortening everything. On a shrub in reasonable condition this is a fifteen minute job once a year and it never needs to become a rescue operation.
Stand back twice while you work. Viburnum has a naturally open, layered outline, and it is easy to lose it by taking too much from one side because that side is easier to reach.
- Take out the dead, damaged and crossing wood Before deciding anything about shape, remove stems that are dead, broken, rubbing against a neighbor or growing back through the middle of the plant. This alone often reveals that no further pruning is needed this year.
- Identify the oldest stems The oldest stems are the thickest, the grayest and the most heavily branched at the top, and they carry the fewest flowers for their size. Mark two or three of them mentally before you cut anything, so you are not choosing with a saw already in the wood.
- Cut them out at the base Take each chosen stem right down to within a couple of inches (5 cm) of ground level, or to a strong low side shoot. Do not leave a tall stub: it will not break into useful growth and it becomes a dieback entry point.
- Shorten anything that spoils the outline If a stem is leaning out of the shape, cut it back to a side branch that is growing in the direction you want, not to an arbitrary length. Cutting to a junction keeps the natural look and avoids the tuft of upright shoots a random cut produces.
- Stop at a quarter of the plant Removing more than about a quarter of the stems in one season pushes the shrub into producing vigorous non-flowering growth. If the plant needs more than that, spread it across three years.
The winter cut that removes next year's beetle
There is one more pruning job, and it has nothing to do with shape. Viburnum beetle lays its eggs in late summer into the current season's shoot tips, in short straight rows of small pits capped with a dark crust of chewed wood and excrement. Those eggs sit there all winter in plain view on bare deciduous stems, and every one of them is a larva that will strip leaves in spring.
In the dormant season, before the buds move, go over the plant and look at the undersides and upper surfaces of the youngest, thinnest shoots. The egg rows are unmistakable once you have seen them: a neat line of raised brown dots, five to ten in a row, running along the last few inches of a shoot. Cut off the affected tip below the lowest pit and burn or bin the prunings rather than composting them.
This one job does more to control the beetle than anything you can spray, because it removes the population before it hatches, at the only point in the year when it is both visible and stationary. On a badly infested shrub you may take off a lot of tip growth; that is an acceptable trade for a year without lacework leaves, and the flowering wood lower down the stems is unaffected.
Note the winter date you did the egg check and roughly how many shoot tips you removed. Two or three years of that record tells you whether the beetle population on your plant is falling, and it is the single most useful thing to have written down about a viburnum.
What not to cut, and how to recover a sheared shrub
Never cut a viburnum with hedge shears if you want flowers, and never cut the whole shrub back hard in one go on the assumption that it will bounce back. Most viburnums do regenerate from old wood, but a hard cut costs two full seasons of flower and produces a mass of upright water shoots that then need thinning themselves.
To bring back a shrub that has been sheared for years, stop shearing entirely and switch to stem removal. In the first year take out a third of the oldest stems at the base and leave everything else, even though the outline will look uneven. Repeat in the second and third year. By the third season the plant is made up of young stems that have never been cut across the tips, and it flowers properly again.
Resist the urge to tidy the new growth that comes up from the base during this recovery. Those shoots are the flowering wood for two seasons ahead, and shortening them puts you straight back where you started.