Q: I would appreciate tips on how to grow rosemary and thyme in an Irish garden. I have managed to get other herbs to grow but no luck with those. I know they need sandier soils so that’s probably my issue? L.K.
A: While many of our most popular culinary herbs are classed as short-lived annuals or biennials, rosemary and thyme are what are called shrubby or woody perennials that will live for several years (often a lot more) if and when given the right growing conditions. I say “if and when” because as you’ve discovered, the challenge for Irish gardeners growing these drought-tolerant, heat-loving, evergreen Mediterranean natives is our cool, damp, rainy climate. Planted somewhere that they’re starved of sufficient heat and sunlight, and/or their roots are sitting in cold, wet soil, they’ll almost inevitably give up the ghost. And who could blame them.
To grow them well, give them a really bright, sheltered spot and a light, free-draining, not overly fertile, neutral to alkaline soil ideally leavened with plenty of sharp horticultural grit. If your garden is inclined to winter wet then consider growing them in a raised bed alongside other heat-loving, aromatic herbs such as sage and fennel. Raising the soil level by as little as 25cm will be enough to dramatically improve drainage and protect their vulnerable roots systems.
These herbs can also be successfully grown in containers, especially if you add horticultural grit to the soil-based compost and lightly mulch the surface with a further top-dressing of horticultural grit or fine pebble. But make sure to use a container with adequate drainage holes, always gently raise the base of the container off the ground with a few bricks or pot “feet” to ensure good drainage and always position it somewhere well away from dripping overhead gutters.
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Container growing aside, thyme also grows well in a gravel garden, along the edges of a sunny path or in generous gaps between paving stones. To keep it bushy, compact and productive, lightly prune in summer after it’s finished flowering using a sharp scissors to gently shear the stems back by a third to young, leafy growth rather than to old wood which won’t reshoot. Many different varieties are available, some with nattily variegated leaves, but you can’t beat the simple species (Thymus vulgaris) for hardiness and vigour.
As a much larger, more spreading shrub that can slowly reach a height and spread of 1.5m-2m, rosemary (Rosmarinus officinalis) needs more space, even if you decide to grow the prostrate version (Rosmarinus officinalis ‘Prostratus’). This frost-hardy plant also needs a sharply drained soil and the protection of a sunny south or west-facing wall to its rear to shelter it from icy winds.
Finally, bear in mind that neither rosemary nor thyme happily tolerate repeated heavy-handed harvesting, especially in a lopsided way to the easiest to reach part of the plant that is closest to hand. Instead the trick is to pick little and often, harvesting in a way that is respectful of the plant’s growth habit and silhouette so that it remains both productive and healthy. As fresh growth naturally comes to a halt outdoors in autumn, it’s also well worth growing a few container-grown thyme and rosemary plants under cover of a glasshouse, polytunnel, conservatory or bright, cool porch/windowsill. An occasional liquid feed will help to keep these container-grown plants happy but go very gently on the watering.