Tried-and-trusted techniques to help plants to grow back more strongly than ever

There are certain nuggets of good garden advice, so counterintuitive that they seem to make absolutely no sense at all. Why on earth, for example, should we pinch out the growing tip of a perfectly healthy seedling or plant stem as a way of encouraging it to fatten up? “Leggy”, after all, is usually a term of admiration rather than opprobrium. Likewise, why would pruning back perfectly healthy shrubs — essentially cutting away their lovely strong shoots and branches- possibly make them bushier and more floriferous when surely it should result in the exact opposite? In the same vein, why is it a good idea to shear back lavender plants that are only just beginning to fluff up again after a long, dark and dismal winter? And what, oh what, is the Chelsea chop, which sounds to the uninitiated like some form of plant abuse?

The simple answer to all of the above, strange as it may sound, is that pinching out, pruning, and cutting back are all tried-and-trusted gardening techniques, which rely on a plant’s innate urge to react to physical injury, by growing back more strongly than ever.

When you pinch out a cosmos seedling or the young stem of a dahlia, for example, you remove its soft-growing tip. This growing tip is very rich in growth hormones and is responsible for a process known as apical dominance, which directs growth upwards rather than outwards. The seedling or plant’s reaction to this physical injury is to quickly throw out multiple side-shoots positioned lower down its stem at intervals along its leaf axils, an ingenious survival mechanism that plants growing in the wild use as a means of surviving damage from grazing animals or harsh weather. So by deliberately inflicting this very targeted injury, you’re using this natural defence mechanism to your advantage to produce a much more resilient, bushier, floriferous specimen.

The same goes for thoughtful, selective pruning which can be used to shape and train plants as well as to positively affect their flowering display. In this case, good timing is everything. For example, some (but not all) spring-flowering woody shrubs benefit from being pruned immediately after their seasonal display of blooms has come to an end. Examples include: forsythia; flowering currant (ribes); kerria; chaenomeles; exocharda; philadelphus; and very early-spring flowering varieties of lonicera. Prune them carefully at the end of spring and you encourage the plants to produce an abundance of young, healthy growth with the potential to flower abundantly the following year. Conversely, if you leave it to late summer, you’ll be removing a lot of that same precious young healthy growth essential for next year’s flowers. Meanwhile, any subsequent new growth produced as a result of your poorly timed pruning won’t have enough time to ripen and develop nascent flower buds before the arrival of winter. So the result will instead be a poor display of flowers the following spring.

How Ireland drinks now: Teetotallers, wine-o’clockers and ex-drinkers

---

Mark Moriarty: two 15-minute meals made easy, with a few simple tricks

---

Every time I travel abroad I am gripped by the fear that Ireland is making a fool of me

---

A personal housing plan: Move to Newry, buy for €150,000 less, commute to Dublin

---

That’s not the case, however, when it comes to cutting back late summer-flowering perennials in late spring, a useful way of delaying their normal flowering period by an average of two to three weeks. Known as the Chelsea chop, because its timing normally coincides with the staging of the annual RHS Chelsea Flower Show in London in late May, this technique allows gardeners to extend the floral abundance of summer. It also results in a shorter, stockier plant that’s much less vulnerable to being damaged by heavy rain or strong winds. Many (but not all) later-flowering species of perennials are suitable for this particular technique, including anthemis, artemisia, echinacea, helianthus, helenium, phlox, monarda, nepeta, solidago, upright forms of hytotelephium (formerly known as sedum), and cota (formerly known as aster).

[ ‘We call Mount Congreve Ireland’s global garden because there are 10,000 different plants here’ ]

Typically the plant is cut back by a third, but it’s also possible to tweak this technique to further extend the flowering period by, for example, cutting a portion of stems back by half, another portion by a third, and then leaving the rest intact. For gardeners planning a holiday away at a time when a favourite plant is typically in bloom, the Chelsea chop is also an excellent way to postpone the display so that you can enjoy it on your return.

Many kinds of late spring/early summer flowering perennials also benefit from being cut back hard after they’ve finished flowering, to encourage fresh healthy growth and avoid leaving annoying holes in a mixed border. Watered well, very lightly mulched, and then liquid fed every fortnight, they’ll also often produce a later, smaller, second flush of flowers. Examples include: geum; delphinium; perennial geranium; stachys; alchemilla, lupin; lamprocamnos (formerly Dicentra spectabilis); and oriental poppies (Papaver orientalis).

[ Tulips: the flowers that get on with almost everything else in the garden ]

Sometimes our reluctance to prune can have unfortunate lasting consequences. A good example is a young hedge, where wariness of causing damage through inappropriate pruning, combined with our natural desire to conserve as much height and plant material as possible, often comes at the expense of bushiness, resulting in a thin, uneven hedge that ends up being sparse and gappy at its base. Good formative pruning in the early years of a hedge’s life prevents this from happening by encouraging the development of a strong, dense stout hedge with lots of bushy side shoots low down on the plants. Formative pruning is especially important for most coniferous hedges (the exception is yew) which even after pruning won’t reshoot from old wood.

The same is true of lavender, a beautiful, floriferous, perfumed shrub and enduring garden favourite whose one great failing is its inclination to quickly become leggy and woody. For this reason, it’s essential to prune it twice a year, first in spring to encourage plenty of bushy, healthy new floriferous growth throughout the summer months, and then for a second time in autumn to keep the plant in good shape. Just never cut into the bare wood of a lavender plant, as this can kill it.

Lavender aside, be reassured that it’s actually quite difficult to kill a plant through overly enthusiastic or poorly timed pruning. Most will survive, even if it takes them a few years to recover in terms of their silhouette and flowering display. On the other hand, the rewards of well-timed and carefully executed pruning are many and well-proven. Just take the time to identify the particular species and always avoid harshly pruning very old shrubby plants which are far more inclined to resent being harshly cut back. Known as renovation or rejuvenation pruning, this requires a particularly respectful approach and often a carefully timed pruning regime staggered over several years.

This week in the garden

A beautiful and important evergreen element of many Irish gardens, box hedging and topiary have come under attack in recent years from two very destructive pathogens, one of which is the fungal disease known as box blight (Cylindrocladium), while the other is the insect pest known as the box tree caterpillar (Cydalima perspectalis), both of which become active at this time of year. Along with a fertile, weed-free soil and good ventilation, control of box blight requires the carefully-timed, regular use of Topbuxus as a foliar spray from March to October. Control of box tree moth requires the use of species-specific pheromone-based box tree moth traps combined with nematode controls such as Nemasys Natural Fruit & Veg, Lepinox Bacillus Thuringiensis and Anti Rups. These should be very thoroughly applied as a spray to the leaves in dry weather (mrmiddleton.com, howbertandmays.ie, fruithillfarm.com, topbuxus.com).

Dates for your diary

Sunday, May 5th (10am-4pm), RHSI Russborough, Blessington, Co Wicklow, the annual RHSI Russborough Garden Show takes place with a line-up of expert speakers that includes Paul Smyth, head gardener at RHSI Bellefield, Conall Ó’ Caoimh of Ardán Gardens in Howth, Co Dublin, Maurice Parkinson of Ballyrobert Gardens & Nursery, YouTuber and Ireland AM garden contributor Niall McAuley, chef Catherine Fulvio and representatives from Festina Lente in Bray, plus demonstrations, workshops, children’s entertainment and plant and food stalls, see rhsi.ie

Sunday, May 12th (10am-4pm), Huntington Castle, Clonegal, Co Carlow Y21K237, The Rare & Special Plant Fair 2024 with a wide range of plants stalls by more than 30 of the country’s leading specialist nurseries, a plant creche, and free admission plus guided tours of the castle’s gardens, see huntingtoncastle.com

• Join us for The Irish Times Inside Politics podcast live in Belfast on April 10th
• Sign up for push alerts and have the best news, analysis and comment delivered directly to your phone
• Find The Irish Times on WhatsApp and stay up to date