Your gardening questions answered: How can I suppress weeds in my rockery?

Q: I have a garden that’s been landscaped and divided into different levels, with boulders dividing each level and a big high section/wall at the front of the site of these boulders. It’s cost me a lot of time and money to get the most out of the site (it was a very rocky spot in the first place, so at least everything was already here), but my problem is now with things like grass and weeds growing between the boulders – they’re all hidden. Is there any plant/moss or flowers that I can plant between the boulders that will stop everything else coming up, but won’t grow too big and hide them too? And how will I kill what’s already covering the boulders? KE, Co Limerick

A: I’m sorry to tell you that the ability of rockeries or similar structures to harbour weeds with roots systems that are then very difficult to dig out is one of the great design flaws of rockeries or similar structures, and the reason why they can be so challenging to maintain. There is no plant that will effectively completely suppress scutch, for example – a thuggish perennial grass with a very vigorous creeping underground root system that loves exactly these kinds of growing conditions. For this reason, garden designers and landscapers often like to lay a weed-suppressant membrane beneath any rocks during the construction phase, but unfortunately in the case of your garden that’s no longer a possibility.

The non-organic option would be to spray with a weedkiller, but you’d have to do this repeatedly as there’s every chance that with time the weeds and wild grasses will find a way to colonise the ground again. A better, more nature-friendly, albeit more time-consuming option – the time-honoured one – is repeated, persistent hand-weeding of the affected areas, along with cutting back down to ground level of any weedy growth whose creeping roots systems beneath the rocks just aren’t reachable. This will gradually weaken the plants’ hold over time.

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Once you have the ground around the boulders clear of weeds, I’d suggest laying sections of weed-suppressant membrane as close as to the edges of them as possible, securing it in place with plastic pins designed for this purpose, or U-shaped lengths of sturdy wire. Then cover it with a layer of ornamental pebble or sand.

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You can strategically plant into this membrane using some low, ornamental plants to give you plenty of seasonal colour and foliage interest by temporarily pulling back a small section of this ornamental pebble or sand, and then cutting an X into the membrane with a sharp scissors to allow you to make a planting hole in the ground. If the soil is poor, work some garden compost and horticultural grit into the planting hole to improve it. Once the plant is in position, water it well, pull the cut parts of the membrane back quite closely down near the base of the plant (but never touching it) and then re-cover the membrane with the pebble/sand.

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Low, creeping plants that enjoy these kinds of sunny, sharply-drained growing conditions and are commonly used in rockeries include varieties of helianthemum, saxifrage, dianthus, aubrieta, campanula, iberis, cranesbill, pulsatilla, snow-in-summer (Cerastium tomentosum), creeping phlox, erodium, shrubby sages and many kinds of dwarf flowering bulbs.