HELEN YEMM THORNY PROBLEMS

This week: confusion over compost, weigelas in winter and a rose in need of some radical renovation

Following an internet quest for topsoil or loam for a new garden, I have taken delivery of something that is described as “heat-treated vegetable matter”. It is light, fluffy and weed and stone-free. All plants seem to be OK so far, but I do wonder if I should add something to this compost? I would value your opinion.

RICHARD CLARK – VIA EMAIL

It sounds as though you have a load of recycled green waste – certainly neither topsoil nor loam. This green waste compost is very high in nitrogen, and is a useful, weed-free enricher of tired garden soil, but it can induce “sudden rush of blood to the head” plant growth if used to excess and “undiluted”. I hope you incorporated it well into your existing soil before you planted.

Your instinct is right: what your imported compost lacks is any soil-like “body”. A good place to start is to add more or less equal quantities of gritty sand to ensure good drainage and bulky humus-creating leaf mould or composted straw to increase its moisture-retaining qualities.

I would also do some simple soil tests that will tell you about your soil’s acidity/alkalinity (pH) as well as the presence of all three vital nutrients: nitrogen, phosphorus and potassium.

Last year, my mature Weigela ‘Bristol Ruby’ flowered really well. This year, after pruning hard back in the winter, it has only in the past few weeks started to flower. Is this normal and what do I do with it this winter?

ALLAN DOWSON – VIA EMAIL

I suggest that the reason your weigela didn’t behave as expected (i.e. flower

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in late spring), is primarily down to your winter pruning. Shrubs that produce flowers in spring and early summer should really be pruned immediately after flowering. The new branches produced thereafter are the ones that should (had they not, in your case, been cut off) carry flowers the following spring.

I also suggest that the autumn flowers produced were encouraged by the hot summer and mild autumn following your winter pruning: the plant seems thoroughly confused. I would now prune out a few branches sparingly, saving as much of the growth that has not flowered to ensure some floral joy next spring.

I acknowledge that this particularly dense form of weigela is a confusing subject: a lot of its new growth is produced on tips of shoots

that are still flowering. Yet, it is still best to stick to one’s guns and cut back the faded-flowered branches around June: even if there is none in evidence, new, lower-down growth will be forthcoming.

Failure to prune means that the shrub becomes increasingly ugly and congested and flowers increasingly poorly, which is when a winter pruning blitz becomes a temptation.

DRASTIC CUTSHard pruning with a bit of TLC can get roses flowering again