Black radishes, or black Spanish radishes, are not as familiar to most UK growers as the red radishes grown in spring and summer. But these winter radishes can also be a great choice for home growers to grow in their domestic polytunnels, or elsewhere in their gardens. These are one of many other types of radishes that you can grow, including daikon radishes, which we have also done an essential grow guide on.
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What are Black Spanish Radishes?
Black Spanish radishes, Raphanus sativus L. var. niger, are root vegetables in the Brassicaceae plant family. They are closely related to other radishes, and less closely related but in the same family as turnips, and other members of the cabbage family commonly grown as crops in UK gardens.
Like other radishes, these radishes have a sharp flavour when eaten raw, though this particular type of winter radish can also be cooked, and when cooked it has a much milder and more turnip-like taste.
The edible roots of these radishes are white, with a tough, black or brownish skin. They can be round or more elongated in shape, depending on the specific variety. These radishes are much larger than the red radishes commonly grown in many gardens – typically they will grow to around 7-10 cm in length or diameter.
Why Grow these Radishes?
There are a number of reasons why it is a good idea to grow these radishes. For one thing, they are moderately hardy, and can cope with low (but not freezing) temperatures. So with a little protection they are relatively easy to grow here over the winter months.
And growing through winter as well as in the summer months can help you to take your home growing to the next level and provide for yourself and your household year-round.
The strong taste of these radishes means that they are not to everyone’s taste to eat raw. Though they are a healthy addition to a salad and can be great when used in this way in moderation. But they are a root crop that can also be added to soups, stews etc. and when cooked, they have a much milder flavour.
These black radishes are very good for us, and consuming them is said to have a number of health benefits. Black radish contains glucosinolates and have been studied for a number of potential uses in natural medicine.
Where to Grow Black Spanish Radishes
Black Spanish radishes are smallish and compact plants that can be grown in a range of different locations in a garden. They have an RHS hardiness rating of H2, meaning that they can survive low temperatures but not being frozen.
They therefore can be grown outside in only the mildest regions over winter, unless they are covered with cloches or given some other form of protection. They grow extremely well in an unheated polytunnel or greenhouse however, where temperatures that remain just above zero can be maintained.
These radishes are a cool season crop and can bolt (rush to flower and set seed) when the temperatures are too high. These can potentially be sown at other times but are generally considered to be winter radishes.
These radishes should be grown in full sun, and receive around 6-8 hours of sun each day. They will do best in a soil that is well-drained and loamy, rich in organic matter, and ideally with a pH of between 5.9 and 6.8. You can grow them in the ground, in raised beds, or in containers, since they don’t need a great deal of space.
Black Spanish radishes can be grown with other brassicas in crop rotation. They like similar conditions to cabbage family plants like kale or sprouting broccoli grown over the winter months and I tend to find that they grow well alongside them. They can sometimes serve as a trap crop for certain brassica pests and so make a good companion plant.
These winter radishes can also work well alongside other root crops, like turnips. They are good for breaking up the soil over winter ready for a sowing of carrots in the spring. Bunching onions make a good companion crop for both these radishes and for the carrots that follow them, helping to keep pests at bay.
Sowing Black Radishes
Black radishes are very easy to grow from seed. They can either be sown indoors and then transplanted to their final growing positions while still small, or direct sown where they are to grow.
When to Sow
Black radishes are typically sowed in August or September and harvested later in autumn or over the winter months. This crop will typically take around 40-55 days to mature to the point where it is usually harvested.
How to Sow Indoors
Seeds can be sown indoors into seed trays, small pots or soil blocks. The seeds should be sown to a depth of around 1/2cm to 1cm. They should be pricked out and potted on if necessary, then transplanted into their final growing positions when they are large enough to handle, with a few true leaves.
How to Direct Sow
Seeds can also be sown directly into the ground or the raised bed or larger container that has been prepared to accept them. This avoids any potential damage during transplantation, though can lead to vulnerable young plants being picked off by pests before they have the chance to grow.
Seeds should be sown to the same depth as above, and sown thinly, aiming for a spacing of 5-10cm between plants, or more if larger roots are desired.
Caring for Black Spanish Radishes
These are not particularly high-maintenance plants but of course, some care is required. Generally speaking, plants in containers will require a little more care and attention than those growing in a larger bed or in the ground.
Water consistently throughout the growth period to keep the soil consistently moist but not waterlogged. Always make sure that excess water can drain away freely. Water availability is especially important during the period when the root is swelling, though these are not a particularly high-water-need crop.
As long as the soil or growing medium is fertile, and you apply an organic mulch around your plants, additional feeding will not generally be required to get good sized roots from your black Spanish radishes. However, when growing in containers, feeding every now and then with a balanced, organic, liquid plant feed may be beneficial.
If you have direct sown your black radishes, you may wish to thin the crop when the roots are still relatively small. Doing so, taking every second radish for salads or cooking, will allow the remaining roots to grow to a larger size.
Looking out for pests is also important. Some pests that can bother this crop include flea beetles, cutworms and root maggots of various kinds. Aim for a rich biodiversity in your garden and no one pest species should have numbers that get out of control.
Harvesting Black Spanish Radishes
Both the leaves and the roots of Black Spanish radishes can be taken and eaten at any size and age. In fact, the whole of the plant is edible.
But most frequently, these radishes are harvested when the roots swell to around 7-10cm in diameter or length, when the top portion of the root can be seen poking above the surface of the soil.
To harvest, round radishes can usually simply be pulled by hand, while those with longer roots may best be harvested with the aid of a garden fork levered carefully below them to lift them from the soil.
You may wish to leave some radishes in the ground, however, as letting the plants flower and set seed means the plants will produce an abundance of seed pods, which are a delicious secondary yield – like those on radishes sown in the spring.
The roots can be left in the ground where they are grown in a suitable position and simply harvested as and when required. Just look out for slugs and other pests that may cause damage if roots are left in the ground for too long.
Growing Radishes in a Polytunnel
We highly recommend growing black spanish radishes in a polytunnel, since this will provide the right environment and conditions for the vegetable to grow in. The controlled climate ensures steady growth, protecting them from harsh weather conditions. As they mature, allowing radishes to go to seed can have multiple benefits, from attracting beneficial insects to providing a source of edible seed pods. As a polytunnel gardener, you should also try letting your radishes go to seed.
Using Black Spanish Radishes
Black Spanish radish skins are typically removed before use. You can then finely slice or grate the root for use in a salad. Young leaves, young flower clusters and seed pods can also all taste good in salads.
But for me, over the winter, mature roots are most useful, these can be sliced and peeled and added to a range of soups and stews, where they have a mild and delicate flavour.
Be sure to check out other vegetables to grow in September.
Can I eat the leaves of Black Spanish radishes?
Yes, the leaves are edible and can be used in salads or cooked like other greens. They have a peppery taste similar to arugula.
Are Black Spanish radishes spicy?
They have a more robust and spicier flavour compared to many spring radishes, but the spiciness can vary based on growing conditions and maturity at harvest.
Petruzzello, M. (n.d.) List of Plants in the Family Brassicaceae. Britannica. [online] Available at: https://www.britannica.com/topic/list-of-plants-in-the-family-Brassicaceae-2004620 [accessed 25/08/23]
Adjmera, R., (2022). 12 Unique Types of Radishes. Healthline. [online] Available at: https://www.healthline.com/nutrition/types-of-radishes [accessed 25/08/23]
Castro-Torres, I.G., De la O-Arciniega, M., Gallegos-Estudillo, J., Naranjo-Rodríguez, E.B. and Domínguez-Ortíz, M.Á., (Year of Publication). Raphanus sativus L. var niger as a source of Phytochemicals for the Prevention of Cholesterol Gallstones. Phytotherapy Research. [online] Available at: https://onlinelibrary.wiley.com/doi/10.1002/ptr.4964 (Accessed 25/08/23]
Elizabeth Waddington is a writer and green living consultant living in Scotland. Permaculture and sustainability are at the heart of everything she does, from designing gardens and farms around the world, to inspiring and facilitating positive change for small companies and individuals.
She also works on her own property, where she grows fruit and vegetables, keeps chickens and is working on the eco-renovation of an old stone barn.
To get in touch, visit https://ewspconsultancy.com.