Thorny problems: do I need mulches, manures and fertilisers?

Fertilisers contain essential nutrients (and minerals), in various ratios for specific purposes in order to bolster plant growth. Most gardeners are now familiar with the fact that nitrogen encourages leaf growth, potash boosts flower production, while phosphates encourage root development and strength. Fertiliser packets reliably carry the information about nitrogen, potassium and phosphate (NPK) content that we need to know.

So-called balanced fertilisers (such as traditional, organic, blood fish and bone or inorganic Growmore), are those that contain more or less equal quantities of all three nutrients.

Finally, I should mention that soluble and liquid fertilisers and foliar feeds of various kinds are useful, being quickly absorbed by plants under stress.

Box in containers

Our 10-year-old potted box plants, nurtured from infancy, seem to be merely hanging on to life. I suspect they need potting on into larger pots but how big should these be, and what sort of growing medium should I use?

Sue Jarvis, via email

Although you dont give me much to go on here, after 10 years even miserable box plants should have developed some sort of root ball and acquired a certain maturity. I hope you have not just been watching them eke out a half-life, undernourished in one- or two-litre plastic pots of exhausted multipurpose compost.

Here is a rule of muddy thumb: a 10-year-old plant of Buxus sempervirens 'Suffruticosa (the small variety used for low hedging and potted topiary), presumably clipped each June into a shape of some sort as is the convention, would need to be in a pot about 18in (45cm) wide and deep. Box needs perfect drainage and rich, gutsy soil John Innes no 3 with added coarse compost to soften it up a bit.

Every spring they will need to have the top few inches of their compost replaced or revitalised with some slow release fertiliser suitable for shrubs, and a seaweed liquid feed is useful in mid-to-late summer.

Very old, large box plants may need repotting/root pruning a few years down the line, but that is a little story for another day if anyone wants to ask about it.

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Thorny problems: do I need mulches, manures and fertilisers?