Welcome to Top of the Crops - today, you will learn how to grow rosemary in a polytunnel. For more gardening insights, be sure to check out our blog Polytunnel Gardening too!
Rosemary is a popular culinary herb with a range of other uses and is a popular plant for herb gardens in the UK.
In wetter and cooler regions of the UK, and for exposed gardens, it may be easier to grow rosemary in a polytunnel, undercover, where this Mediterranean herb can thrive in warm, dry and sheltered conditions.
Rosemary, Salvia rosmarinus, formerly known as Rosmarinus officinalis, is a type of sage, and will grow well alongside members of the same family and genus, along with other plants from a Mediterranean climate.
This is a shrub and a herb prized not only as a pot herb, but also as a companion plant, and as beautiful and fragrant garden ornamental. There are many reasons to grow this attractive, often blue-flowering plant in your garden.
Rosemary has attractive flowers, usually blue but sometimes also mauve or pinkish in hue. It grows relatively slowly but plants can reach up to 1.5m in height when fully grown.
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You can grow rosemary in either a container or direct in the soil. Where you decide to grow can be important to achieving the best results.
Your rosemary should thrive, and be a relatively low maintenance addition to your garden, as long as you pay attention to the basic growing conditions that these plants require, and make sure that the preferred conditions are provided.
Rosemary needs a position in full sun to thrive. It will do best where it receives at least 6 hours of direct sunlight each day.
Rosemary can potentially be grown indoors as well as outside in your garden but when grown indoors it will need a position on a sunny and ideally south-facing windowsill and still, grow lights can still sometimes be necessary to prevent poor and weak growth.
Rosemary requires a suitable soil that must be free-draining and not become saturated. Heavy clay soils are not ideal for growing this plant and so if that is the soil type where you live, you may be better growing rosemary in a container.
Rosemary has deep roots and so will do best where the soil or growing medium is deeper. It prefers a pH of between 6 and 7 (neutral or slightly acidic).
Remember, rosemary is a Mediterranean herb, which grows naturally in a Mediterranean climate zone. It will therefore do best where we can provide conditions as close as possible to those it would experience in its native environment. Reasonably warm temperatures and moderate humidity levels are preferable.
Rosemary can be grown from seed, which should be sown in around May, though in the UK it is far easier and more usual to grow this herb from small plants, or rooted cuttings, which are available in most garden centres and plant nurseries.
If you do decide to sow rosemary seeds, note that it will take some time for the seeds to grow into plants of a reasonable size, so patience will be required. You will not usually be able to harvest until at least 2-3 years have elapsed.
Sow seeds into a a suitable, free-draining seed starting potting mix indoors in the spring. Once the seedlings are large enough to handle, prick them out and pot them on into their own individual containers. Note that germination can be slow, so do not give up on the seeds too quickly.
Place them in a warm, bright location, watering frequently but always making sure that excess water can drain away freely. They can be planted into their final growing positions outside or in a polytunnel when they are around 10cm tall.
The process of planting out rosemary is the same whether you have grown the plant yourself from seeds or cuttings, or purchased the rosemary plant from a garden centre or plant nursery.
The best time to plant out rosemary is in the spring, though you can plant out in summer or autumn too, as long as you avoid hot, dry spells and water in well and continue to water well during establishment.
Make sure that you have chosen a suitable growing area, and prepared that area to provide the right growing conditions for rosemary plants to thrive. If growing in a container, make sure that the container is large enough to accommodate the existing root system of the plant, and has sufficient drainage holes at the base.
Rosemary is not a herb that requires a great deal of care once established. It can be an excellent low-maintenance plant in the right location.
Rosemary plants self-pollinate and also attract a range of beneficial pollinators to your garden. They are beloved of bees and also attract a range of other insect species that we want in our gardens.
Rosemary grown in a container should be potted up into a suitably free-draining container filled with a suitably free-draining growing medium, with just a little space around the existing roots. Each spring, you should repot with fresh potting mix to prevent any problems.
Rosemary is fairly drought tolerant and so you should water only sparingly and make sure that you do not overwater, especially during the colder months. Waterlogging is one of the most serious problems for this plant and the most important thing to avoid.
It can be beneficial to prevent waterlogging around the base of the rosemary plant by placing gravel around the base. This can also help to protect rosemary from winter cold. You might also use a mulch of homemade compost around the base of the plant, but take care not to allow this to touch the base of the stems or rotting can occur.
If you are growing rosemary in the ground or in a raised bed, feeding will not usually be required as rosemary can grow even in poor soil. However, when growing rosemary in a pot, it can be beneficial to feed the plant with an organic, balanced plant feed after the flowering period finishes each year.
To keep rosemary plants healthy and well-shaped, it is a good idea to trim back the stems after the blooms fade. If you do not do so the plants can have a tendency to become leggy and the stems will become woodier further up from the base.
Rosemary is hardy down to around minus 5 to minus 10 degrees Celsius. But during hard winters, you may need to provide a little extra protection for your rosemary in some parts of the UK.
Raise container grown plants onto pot feet to guard against waterlogging and, if you have grown rosemary in a polytunnel bed, apply a thick mulch to protect plants in the ground.
In the coldest parts of the UK, you may also have to cover the branches of a young and tender rosemary plant with horticultural fleece.
Rosemary can be propagated in a range of different ways. It can be propagated by seed, from softwood heel cuttings in spring, semi-ripe cuttings in late summer, or by layering.
Regularly picking the needle-like leaves and tips from your rosemary bush should help to keep the plant well-shaped and productive. Simply take the tips as required throughout the year. Rosemary can be used fresh in recipes, in a bouquet garni or dried for later use.
Rosemary is easy to dry for longer-term storage. Simply place it in a vase or other container, or hang it in bunches to dry out within your home. Then once it is fully dry, it will be brittle and can be crumbled up and put into jars or other receptacles for later use.
This intensely flavoured herb is only used sparingly in the kitchen. It is typically used in smallish quantities in a culinary context. Interestingly, however, rosemary is not only useful as a pot herb. It is also useful medicinally, and has a range of other uses around a home.
For example, rosemary can be beneficial in self-care cleaning and beauty regimes, and also used as an ingredient in homemade natural household cleaners. It also yields an essential oil with a wide range of uses and is even said to boost brainpower.
Rosemary varieties with an award of garden merit from the RHS include:
Most of the problems that arise when growing rosemary do so because of an issue with the environmental conditions or care. In particular, overwatering or poor drainage leading to waterlogging can cause roots to rot and rosemary to die. Low temperatures are usually less of an issue, but can still be problematic for new growth, and for roots of potted rosemary plants.
When it comes to pests, you should look out for scale insects, and rosemary beetles. However, these plants do not have serious pest issues very often, and in fact they can help confuse, distract or repel pests of other plants and crops.
Rosemary will, remember, grow well alongside other drought-tolerant Mediterranean herbs such as thyme and lavender and oregano, can therefore also be used in a dedicated herb garden area. But you might also grow it in a pot or around bed edges close to other crops.
Rosemary is also said to be a useful companion plant for brassicas, carrots and beans in the vegetable garden as it can help to deter a wide range of pests that can damage those crops.
A polytunnel can make it easier to regulate water and make sure waterlogging or over-saturation does not occur, as it can outdoors in our climate. But make sure the polytunnel is well-ventilated and that humidity does not rise too high, as many issues can arise if there is not enough airflow around rosemary plants and if humidity is too high.
McCulloch, M., (2023) 14 Benefits and Uses of Rosemary Essential Oil. Healthline. [online] Available at: https://www.healthline.com/nutrition/rosemary-oil-benefits
