One of the most popular home ingredients, achieving high-quality onions in your own greenhouse is a great idea for many households. Onions are relatively easy to grow, making them an ideal first venture for amateur gardeners. Due to their hardiness, onions are the perfect spring plant meaning that, come summer, you can have access to delicious onions in the comfort of your own home. Want to find out how to grow onions in a greenhouse? Read on for our advice and top tips.
How to grow onions in a greenhouse
Planting onion sets instead of seeds is the easiest way to achieve the best onions in the fastest time possible, as they mature faster and require less work, making them ideal for the amateur gardener. An onion set is simply an onion bulb that is partly formed- growers plant onion seeds closely together to create small bulbs that are perfect for cold-weather growing. Our top tip? Select onion sets that are no more than ¾ of an inch – anything smaller and they’ll take longer, while larger sets may develop too quickly.
To plant onions in a greenhouse, follow these simple steps:
Choose the right time of year: for the best results, onion sets should be planted from mid-march to mid-april.
Select the optimum spot: in order to grow, onions require around 10 hours on sunlight a day, and so should be planted in a sunny yet sheltered site with well-drained soil – this means that, when planting onions, you should opt for deep pots, containers or raised beds to offer the roots maximum growth.
Prepare the soil: it’s a good idea to mix a balanced fertiliser into your soil a few weeks before you plan to plant a new bed and soil should be raked before planting in order to break down any large lumps. For optimum growth, you’ll need to use the correct soil type in order to properly nourish your onions as they grow as they need soil that is well-drained, loose and rich in nitrogen to thrive, and will not grow well in soil that is too acidic.
Space out your onion sets: as onion bulbs can grow quite large, you’ll need to space out your onion sets when planting them – roughly 10cm or 4in apart is best. First, using a piece of string or a bamboo cane to keep the line straight, mark out holes in the soil in regular intervals to avoid damaging the onion roots and place your onion root first into the soil making sure to leave about a foot between your rows. Finally, gently gather soil around the set so just the top of the onion is visible and gently pat-down.
Caring For Your Onions
Onions are very easy to grow and require very little attention. As with many vegetables, you must keep your onions moist by watering them regularly – just be careful not to overwater them – while any flower spikes that begin to grow should be gently removed.
Roughly towards the end of July, stop watering your onions so that they dry. This is so that when the bulbs have swollen they have sufficient time to mature. For optimum results, it’s also a good idea, at this time, to gently brush back the soil from the top and neck to allow for optimum sunlight.
Onions can be susceptible to the spread of diseases such bacterial and fungal spores, which can cause decay further down the line, especially if they have been overwatered or the greenhouse is particularly moist. To prevent this, gardeners should practice crop rotation to reduce the risks and can also stop spores from multiplying by spraying a fungicidal spray on the leaves weekly.
Harvesting Your Onions
You should aim to harvest your green onions roughly 30 days from planting a set, when the plants are at least 1 inch in diameter and 6 inches tall although this does depend on the plant variety and your individual conditions. Once you have harvested your onions, you can leave them to dry out for around a week – spread your onions out on a clean dry surface and leave in a well-ventilated yet dry location to fully dry the outer skins, prevent any mould from forming and allow them to ripen fully.
You can use your onions straight away to create delicious dishes, or store them for future use. If storing, cut off the root and remove any loose skin from the onion and place in a cool dark place out of direct sunlight for up to a year.
Searching for more advice on growing vegetables in a greenhouse? We have a wide range of articles to help you grow a variety of vegetables – from growing courgettes to cucumber, sweetcorn to peppers, you’re sure to find enough to keep your greenhouse well-stocked.
Thank you for taking time to read our ‘How to Grow Onions in a Greenhouse’ guide. As we have seen, onions are relatively easy to grow, making them an ideal first venture for amateur gardeners. For more information, the RHS have an in-depth article on the topic.