Welcome to Top of the Crops - today, you will learn how to grow blank in a polytunnel. For more gardening insights, be sure to check out our blog Polytunnel Gardening too!
Pelargoniums, also known as tender geraniums, are among the most popular summer bedding plants and are often used as houseplants as well.
These are an excellent choice to grow in a polytunnel, either in the ground, amid other plants, in a raised beds or in containers. If the polytunnel is heated they may be able to remain there all year. Otherwise, they can be placed in a polytunnel in summer then moved indoors over the winter months.
Pelargoniums originally hail from southern Africa but they are popular in temperate climate gardens including here in the UK because they flower over a long period and flower attractively over quite a long period of time.
There is a little confusion surrounding pelargoniums since, though they are often called 'geraniums', these are a completely distinct plant to the true, hardy herbaceous geraniums and have not, in fact, been categorised in the same genus for quite some time.
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A huge variety of Pelargoniums is now available because selective breeding has developed a great many different types of Pelargonium that differ in their blooms and in other features. The main types are:
Zonal Pelargoniums
Ivy-leaved Pelargoniums
Fancy-leaf Pelargoniums
Regal Pelargoniums
Scented-leaf Pelargoniums
Angel Pelargoniums
Decorative Pelargoniums
Stellar Pelargoniums
Unique Pelargoniums
Many of the above Pelargoniums are commonly grown for summer colour. You will be able to find ones that have flowers in a number of different hues. Fancy-leaf types are also prized for their foliage, which can also come in a range of hues.
Some pelargoniums, especially within the scented-leaf category, but also within other type groups, are also known not only for their visual appeal but for their scent. The leaves of many Pelargoniums have a distinctive fragrance, but the scented-leaf types can be edible and have different scents – smelling like mint, citrus, or rose, to give a few examples.
In addition to looking at visual appeal and fragrance, when choosing a Pelargonium it is also useful to think about the plant's size and habit of growth. Some grow more upright in form, some have a trailing habit (ivy-leaved for example), and some will grow much larger than others.
Pelargoniums are usually purchased as small plug plants or potted specimens to be planted out after all risk of frost is over in the spring. In a polytunnel, you may be safe to plant out from April as long as the polytunnel is frost-free. When growing outdoors it is often best to wait a bit longer, and until early June in Scotland and other cooler regions of the UK.
It is important to remember that these plants cannot tolerate frost and so it is vital not to plant out too early. Remember too that Pelargonium need to be hardened off before they are placed into their final growing positions.
You can plant Pelargoniums into the ground in a suitable location, in raised beds, or in containers, as long as their basic growing needs are met.
The key things to remember are that :
Many Pelargoniums will do best in full sun, though some types tolerate or even prefer light/ partial shade in summer. Indoors, a location with bright but indirect light is usually best.
They also require a sandy soil or growing medium that is free-draining, with a neutral or mildly alkaline pH. In containers, these plants will thrive in a soil-based potting mix.
Pelargoniums might make good companion plants when placed in containers close to vegetable beds, to draw in beneficial insects for pollination and organic pest control.
Though they are tender plants, Pelargoniums are not particularly dainty nor difficult to care for. They are fairly tolerant of heat and drought.
When it comes to watering, it's essential to strike a balance – pelargoniums don't enjoy soggy soil but also don't like to dry out completely. It's best to water them deeply but infrequently, allowing the soil to dry out slightly between waterings to prevent root rot.
During the growing season, typically spring through to early autumn, watering once or twice a week should suffice, depending on the climate and soil conditions. Stop watering before the first frosts threaten and water only very sparingly in the winter months if keeping your plant in a container through the coldest part of the year.
Incorporating a slow-release fertilizer into the soil at the beginning of the growing season can provide a steady supply of nutrients over time, and mulching can additionally help to meet nutritional needs. But plants growing in pots will also benefit from the provision of an organic liquid plant rich in potassium once every fortnight to once a month or so through the flowering period.
A general purpose organic fertiliser may sometimes be applied in the spring if growing in containers or if growth seems poor, though once flowers form, it is important to change to a high potash fertiliser such as a comfrey tea – such as would be used on tomato plants. A feed too high in nitrogen can encourage leafy growth at the expense of flowers.
Remember when growing pelargoniums in the UK that there are several ways to ensure that your plants make it through the winter months and ensuring that you can enjoy pelargoniums again the following year.
Method 1: Overwintering by taking cuttings
Overwintering pelargoniums by taking cuttings is a popular method to ensure the survival of these beloved plants through the colder months. As temperatures drop and daylight diminishes, pelargoniums, also known as geraniums, can struggle to thrive outdoors.
By taking cuttings from healthy parent plants before the onset of winter, gardeners can propagate new plants that will be ready to flourish come spring.
To take cuttings, select healthy stems, remove any flowers, and trim the stem just below a node. These cuttings are then placed in a well-draining potting mix and kept in a bright, warm location indoors or under cover.
Method 2: Overwintering in containers under glass
Overwintering pelargoniums in containers under glass offers a protective environment for these plants during the colder months and is another way to make sure that you can continue to enjoy your plant or plants the following year.
With proper care and attention, pelargoniums overwintered in containers under glass can maintain their vigour, ready to thrive once again when spring arrives.
Method 3: Overwintering as bare-rooted plants
A third method for overwintering involves keeping Pelargoniums as bare-rooted plants. This method involves lifting the pelargoniums.
Gently shake off excess soil from the roots, and trim away any damaged or dead foliage. Once cleaned, the bare-rooted plants are typically stored in a cool, dark, and dry location, such as a basement or garage, with temperatures above freezing but below around 10 degrees C.
In order to encourage bushiness and branching, you should pinch back the growing tips of Pelargoniums in the spring or early summer. This is true for both more upright forms and trailing types.
Pelargoniums generally flower freely and over a relatively long period. Deadheading is a good idea because this should promote further blooms. Let the flowers fade and the plants will often focus on producing seed rather than on further flowers.
Whether or not you should cut back pelargoniums at the end of the growing season depends on how you plan to overwinter them.
As mentioned above, you may keep them as pot plants indoors or in a heated space over the winter. Or you might lift and store them as bare-root plants. With either of these options you should cut back hard in autumn, or in the early spring.
As mentioned above, taking cuttings is one way to propagate your pelargoniums and make sure that you can enjoy the same variety again next year. You can take cuttings any time between spring and autumn and these are fairly easy to root successfully.
Some pelargoniums including some bedding types and some species types can also be propagated from seed, sown indoors or in a heated greenhouse in the late winter. But note that in many cases, the cultivars grown in gardens will not come true from seed and offspring may not at all resemble the parent plant.
If you wish to experiment in growing pelargonium from seed then you should note that these are one of the seeds that needs light for germination.
There are numerous Pelargonium varietals with an award of garden merit from the RHS and which are highly regarded by home gardeners and professional growers alike. Some options to consider are:
Though of course these are only a very small number of the many Pelargoniums that you might consider growing.
Most of the issues that arise when growing Pelargoniums do so because the plants have not been given the optimal growing conditions or care. Issues often arise, for example, because plants get too cold, too wet, or too dry.
Waterlogging can often lead to rotting roots and a range of fungal issues. Inadequate ventilation can make issues like rust or grey mould more likely to take hold. And plants under stress due to environmental issues are more likely to succumb to pest infestations with vine weevils, thrips, root mealybug or leafhoppers, all of which can sometimes be pests of Pelargonium plants.
Remember that Pelargoniums need a frost-free location and so will usually have to go indoors in winter unless your polytunnel is heated.
Choose varieties suited to where you wish to grow them. Some are great for hanging baskets or other hanging containers, for example, to help you make the most of the space you have in your polytunnel.
Ensure good plant spacing, and good ventilation to ensure humidity remains at a suitable level and the chances of fungal issues are not increased.
The scent of pelargoniums may potentially help repel, confuse or distract pest species in or near your polytunnel garden.
Mills, S., (2023) Pelargonium: benefits and uses. Healthspan. [online] Available at: https://www.healthspan.co.uk/guides/pelargonium-benefits-uses/
