Jo Wittingham: We still need to exercise caution, come what May

May can be a tricky month in the garden, because the strong sunlight fuels incredibly fast growth, but nights can still be cold '“ even frosty.

Its time to prune any wayward growth on Clematis montana

Good weather makes any gardener want to get ahead; planting out tender seedlings, moving cosseted specimen plants out of their protection and filling pots with bright, half-hardy summer bedding. But always exercise caution, as a quick cold snap can cause disaster. To be safe, don’t plant out anything which can’t tolerate frost before the end of the month and, if you do, keep an eye on the forecast. If temperatures take a dive, be ready to cover plants with protective fleece or cloches, or move pots under cover.

One way to help get seedlings off to a good start is to harden them off before planting out. This means acclimatising them to outdoor conditions, and is as easy as moving them outside for progressively longer periods over seven to ten days. This applies as much to young plants bought via mail order or from the garden centre as it does to those raised in your greenhouse. Keep a careful eye on potatoes and pull the soil around emerging shoots to earth them up as protection from frost.

Also, be wary of moving tender crops, such as tomatoes and cucumbers, into unheated greenhouses too early. Tomato plants dislike temperatures below 10C, and the fluctuation between soaring daytime temperatures on sunny days and chilly clear nights is not helpful, either. I always keep mine indoors for as long as I can without them getting leggy or pot bound.

Whatever the weather, sow courgettes, squashes, sweetcorn, French and runner beans under cover now, ready to plant out come June. Rows of tougher outdoor seedlings, such as carrots, beetroot and radishes, need thinning out to their recommended final spacings, to allow them to mature to a good size.

Be sure to support sweet peas, peas, and herbaceous perennials as they shoot upwards, and tie-in climbing plants to keep them where you want them. Between hoeing to keep weeds down and cutting the lawn regularly, spare some time for plants whose spring flowers are already spent. Cut back early flowering perennials, such as Doronicum and Pulmonaria, to encourage a flush of fresh growth, and prune wayward growth on Clematis montana. Lift and divide overcrowded primroses and primulas, along with any clumps of daffodils that are too large or badly placed. Remember, however, to leave all bulb foliage to die down naturally to give it time to feed the bulb in preparation for next spring’s blooms.