Dividing and replanting perennials helps you create an attractive and interesting border and the next few weeks are a good time for many plants.

It is all too easy to end up with a randomly planted border filled with impulse buys stuck in to an available space with no coherent design.

Self control and an ice pack gets much better results. I reckon designing a planting theme containing fewer different species and repeating the pattern along a bed is much more satisfying. You use clumps of individual species to form the pattern and this would be impossible with a random jumble.

Dividing existing clumps helps to achieve this repeat pattern. For free. It also controls some exuberant spreaders intent on throttling their neighbours and you can bulk up or refresh other clumps.

Divide once a plant has flowered. This could be now for those that flower in early summer. The soil is moist and perfectly warm, allowing transplants to establish a strong, fresh root system. They can them get going quickly next year.

A lot of gardening advice from the south of England can be confusing. Autumn, presumably September, is often recommended for this but that can be getting too late for Scotland. We have a much shorter growing season with lower light levels so transplants will struggle to get enough root growth before winter.

Because of this short season, we must leave later flowering specimens till next spring, even if they may develop more slowly if the weather is cold.

Bearded Iris is a good, easy candidate for dividing just now. You should anyway do this every 4-5 years to keep a display gorgeous and the crown healthy. As the crown develops, rhizomes grow out from the centre, which then produces few if any flowering stalks.

Don’t divide a young plant that has only a few rhizomes. Make sure the ground is damp and water generously if it’s at all dry to ensure roots won’t break away from the rhizomes when you lift the crown.

Gently tease the fully formed rhizomes apart and away from the original central crown. It will be soft and withered with no leaves coming from it. Only save and replant firm healthy rhizomes, discarding any that break or have a tangle of intertwining roots.

Replant in moist, fertile soil, leaving the top of the rhizome exposed: it needs direct sun to keep firm and healthy. That’s why I always plant irises near the front of a border to prevent their neighbours growing over and covering the rhizomes.

Ajugas are even easier to divide. They throw out little plantlets attached to runners, so you simply detach and replant. Lily of the Valley, Convallaria majalis is another beginner’s candidate. It has rhizomes that travel and branch, sometimes turning up in unexpected places. Just lift and relocate if necessary.

You need a sharp knife for some other plants, like Hemerocallis. I, for one, could never have enough of the likes of ‘Lemon Bells’. Lift the entire clump and remove soil from the roots so you can see where the individual leaf fans join the clump. Push the knife vertically between the fans, trying to cause as little damage to the roots as possible.

Some plants that make fibrous rooted clumps, like Geranium x magnificum, Ranunculus aconitefolius, Bachelor’s buttons, or chives, are a bit tougher to divide and you’ll need to use 2 forks, placed back to back against each other in the centre of a crown. Ease the forks apart like scissors. You might even have to hack through with a pruning saw.

If you need to divide a plant but don’t have space for the transplants, pot them up for planting out later.

Plant of the week

Tomato ‘Dora’ is a small bush variety that produces masses of small, early ripening fruit. Each bright red tomato is slightly pointed giving it a heart shape when sliced in half; great flavour.