Buddleia is one of those shrubs that Irish gardeners are often too cautious with. Left unpruned for a few years it becomes tall, leggy and sparse, a framework of old woody stems with flowers only at the very top where you can barely see them. The instinct when faced with a plant in that state is to cut it back by a third, tidy it up a little, and hope for the best. That instinct is wrong. Buddleia wants to be cut hard, right back to low down, and it responds to that treatment with a vigour that will surprise anyone who has been tip pruning it for years.

The reason you can cut buddleia so hard comes down to how it flowers. Unlike spring flowering shrubs such as forsythia or flowering currant, which produce their blooms on growth made the previous year and should not be cut until after flowering, buddleia flowers on growth it produces in the current season. What that means in practice is that the harder you cut it back in early spring, the more new growth it will throw up between now and summer, and the more flower it will carry come July and August. A plant that has been cut back to forty or fifty centimetres from the ground in March will often reach two metres or more by the time it flowers. The energy is all there in the root system. You are simply directing it.

The right time to do this in an Irish garden is now, in early spring, as the season is beginning to move. You will often see small green buds beginning to break low down on the stems at this time of year, that is confirmation that the plant is ready and that the cuts you make will be met with active growth. If the plant has been neglected for several years and has become very congested with multiple thick old stems, work through it methodically rather than cutting everything at once. Take out the oldest and woodiest stems first, cutting them as close to the ground as you can manage. Then cut the remaining stems back hard, leaving just a few pairs of buds above the base. The whole operation should leave a plant that looks dramatically reduced, almost brutally so, but that is exactly right.

One concern that comes up regularly is whether cutting this hard will mean losing flowers for the entire season. In the vast majority of cases the answer is no. Buddleia cut back in March will flower by July or August as normal. Where you might lose a season of flowers is if you cut very late, into May or June, once the plant has already put substantial new growth on. At that point you would be removing the growth that was going to carry the flowers. Early spring pruning, before that growth gets going, avoids that problem entirely. If you have missed the ideal window this year, it is still worth cutting back. You may get a reduced display, but the plant will be in much better shape for the following year.

After pruning, buddleia benefits from a feed. A general balanced fertiliser worked into the soil around the base, or a biochar based granule which will hold nutrients in the root zone and release them steadily through the growing season, will give the new growth the best possible start. Buddleia is not a hungry plant by nature, but in the years when you have cut it hard and are asking it to regenerate substantially, a feed at this point will show in the quality of the growth and the flower. Beyond that, it needs very little. It is drought tolerant once established, it does not need staking, and it will look after itself through the summer. The main thing is simply to be willing to cut it properly in spring, without hesitation.