Cauliflower & Kohlrabi

Brassica oleracea, Botrytis Group

The word cauliflower is from the Latin caulis (cabbage) and flōs (flower).

Introduction

Cauliflowers are clusters of buds of cabbage flowers, selectively bred to grow into firm curds before the full flowering stage. The word cauliflower is from the Latin caulis (cabbage) and flōs (flower).

They were first written about in Roman times and then were much grown in mediaeval Cyprus. During the 16th century, there are first mentions of Romanesco broccoli, which is actually a cauliflower. They are pale green, while recent breeding has given us cauliflower curds of varied colour.

Cauliflowers are mostly annual, but not exclusively. I have grouped them in this lesson with family relation kohlrabi, because both of these vegetables have a similar time pattern of growth, just for a totally different result!

Cauliflowers (and kohlrabi) grow extremely well in no dig soil, as the gardeners and presenters discovered in 2016–18 at the BBC Beechgrove Gardens in Scotland. They ran a trial of vegetables in dig and no dig beds, and started off feeling sceptical. Soon they were, and continue to be, enthusiastic for no dig, as you can imagine from the photo below comparing cauliflowers – dig on the left and no dig on right.

A very beautiful Graffiti cauliflower, cut in half – this one is not yet fully mature, but is still striking and was super tasty

Jim McColl’s write up of the success of a no dig trial at the BBC Beechgrove Gardens – Aberdeen Press and Journal, November 2017

Graffiti cauliflower cooked two ways – roasted on the left, and boiled with French beans on the right; you’ll notice how the anthocyanin colour has not been washed out by boiling water

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Harvest period

Days from seed to first harvest: 100–150 for annuals, 270 for biennials.

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Best climate is moist, not dry, temperate without extreme heat.

Why grow them

There is something special about seeing the emergence of a small cauliflower in the middle of a huge mass of leaves. Then to watch it turn into a beautiful and tasty harvest.

Cauliflower plants are demanding for the results we desire, and need fertile soil plus temperate weather. They pose a challenge, but it’s worth the attempt for harvests of great flavour, colour and beauty.

Each variety needs sowing at its best times.
They need fertile soil.
They are vulnerable to insects.

When you succeed, you know you are a good gardener.

Suitable for containers/shade?

You can grow cauliflower in shade, but I do not recommend growing them in containers because of their size. The harvest of food, per square metre and time needed, is relatively low, and I see them as a luxury. You may disagree!

Varieties

The nomenclature varies and can be confusing. Sometimes one sees the phrase ‘Italian cauliflowers’, which include Romanesco and all kinds of different coloured cauliflowers. In this lesson, my main differentiation is between annual and biennial types of cauliflower.

There are four common colours: white, purple, orange-yellow and green.

Mid-April – these impressive Aalsmeer cauliflowers are fully formed, sown the previous July

A striking Grafitti in late June, from a sowing in late February

This fully formed, medium-sized Sunset cauliflower was sown in late February, and is now ready for harvest in mid-June

Annual

Read the small print for each variety to check the maturity time, because they all take a slightly different amount of time from seed to harvest. Many good varieties are hybrids now.

You have exotic colour choices, all of which are hybrids. Graffiti is best for a fast-maturing purple cauliflower. Its colour is from anthocyanin and survives boiling in water, unlike purple broccoli which turns green when boiled.

For green curds try Trevi, and for orange I like Sunset. The orange colour comes from beta carotene or vitamin A.

The old open-pollinated variety All Year Round can be sown from February onwards and is reliable, if not remarkable.

Clapton F1 is white and claims resistance to clubroot.

For Romanesco, I have found recently that the old open-pollinated varieties are giving smaller heads. I recommend hybrids such as Celio and Navona.

Romanesco Navona in mid-November from a sowing in June – the head is fully mature; some Kaibroc broccoli on the right

Romanesco here in early January, sown in mid-June; although the head is pretty, it is small – an open-pollinated Natalino

Romanesco weathering a -2 °C/28 °F frost in November, using its own leaves to protect from the chill; below -4° C/25 °F there is some frost damage

Biennial

Walcheren, Aalsmeer and Medallion F1 are examples of the many varieties you can grow to overwinter, from sowings in late June to early July. In seasonal terms that means sow before midsummer, for harvest in early to mid-spring.

Maybach F1 can be sown in October/early to mid-autumn, to overwinter as a small plant in a pot in the greenhouse, and then transplant in March for cropping in May.

Purple Cape F1 is both beautiful and productive, and is also hardy in temperatures as low as -15 °C/5 °F, before the curding stage in early spring,

Aalsmeer cauliflower, originally sown in July, now looking a bit limp in a February frost – the temperature is -9° C/16 °F

The same Aalsmeer on 10th April, now starting to form lovely curds

By 1st May the Aalsmeer has a magnificent head – it’s a fantastic vegetable for the hungry gap

Video

Purple cauliflower, at 65 days from transplanting

Sow and propagate

You need only a few cauliflower plants, therefore indoor propagation is totally worthwhile. Sow sparingly and resist the temptation to plant out too many of the same variety at one time, because you are then likely to have a glut. Cauliflowers from any one sowing will mature at a similar time.

Seeds germinate quickly, but often unevenly – first leaves appear in four to six days.

Sowing conditions

As for cabbage, in temperatures as low as 10 °C /50 °F. However, you see better results when it’s 15 °C/60 °F or higher. Early sowings can be germinated on a windowsill.

Germinating kohlrabi on the left and cauliflower in the middle, sown on 7th July; this is 12th July, and the kohlrabi are ideal for pricking out now, while the cauliflower could be pricked now, or a day later

Medallion cauliflower plants on the left, sown in July and pricked into these modules two weeks previously – they are ready from now to transplant in late July; kohlrabi on the right

Sowing time

See the table above.

Do check the seed packet suggestions, because each variety matures best at a different time of year, from sowing at a particular time. First sowings are from mid-February under cover, and last sowings can be October under cover. There is a mass of possibilities.

For cauliflower harvests in autumn, the last sowing date in temperate climates is late June.
Sowings in and after July need to be biennials, giving time for them to grow into medium-sized plants before winter, for eventual harvest in spring.

Sowing method

You want one plant per module, so either sow in a tray to prick out, or sow two seeds per module and thin to one. For hybrids’ expensive seed, I advise to sow in a tray, then prick out.

After a week or so, prick seedlings into modules of 3–5 cm/1.2–2 in width. They need full light, otherwise stems grow long and thin (‘leggy’). Early sowings can grow in an unheated greenhouse because seedlings tolerate some frost.

Five-week-old cauliflower, already potted from modules a week earlier – this was for a photoshoot! Now they are ready to be potted on to 10 cm/4 in pots

The cauliflower plants safely in their new homes for another eight days – I transplanted them on 20th April

Pot on?

You have options. Either transplant at four weeks from sowing, or pot four-week-old seedlings into a 7 cm/3 in pot. From a very early sowing, potting on may gain an earlier harvest, plus it buys time if the planting bed still has a different vegetable, so is not ready yet for transplants.

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Transplant

Grow in a slug-free bed, with compost the only mulch. Set plants deep in the ground so that all or most of the stem is below surface level. Water in, and make sure the module or pot stays moist for the first week.

Transplant size

As for many brassicas, a plant height of 10–18 cm/4–9 in is good, according to conditions and when the ground is ready.

11th July – new transplants of broccoli, nearest, and Romanesco, in beds still partially occupied by lettuce plants; the cleared lettuce gave two more weeks of harvest, before being twisted out

1st August – Calinaro near the Buckler Leaved sorrel, then Natalino Romanesco, just sprayed with Bacillus thuringiensis; no cover used

Romanesco in early January are cropping now, after the broccoli in front have finished – they gave their main harvest in October and November

Transplant time

From mid-spring to late summer, according to the variety sown, and when.

Spacing

Space at 45–60 cm/18–24 in, depending on how large you want your plants to grow. This means that beds can easily be filled with only a few plants, not always a desirable thing if space is short.

You can reduce the need for space by making just the two sowings I recommend: (1) late winter to early spring, and (2) midsummer.

Cauliflowers’ need for space is mitigated both by their speed of growth, and how they can overwinter. There is time after clearing plants from sowing (1) to grow a lot of other vegetables, and you can plant biennial cauliflowers from sowing (2) after significant early harvests.
Each harvest requires less growing time and fits nicely with other sowings. A contrast is courgettes/zucchini, whose growth period is longer and spans both the main spring planting and summer planting times.

See the spacing between Romanesco cauliflowers, allowing 52 cm/21 in between them

21st July – Medallion, a biennial cauliflower, has been recently transplanted at the far end of the right-hand bed, in the Small Garden

The same Small Garden shown in early January, with the Medallion cauliflowers top right

Water

How often

Once transplants are watered in, they can be left unwatered for much of the time unless it’s exceptionally dry. Once you notice the central leaves folding over an initiating curd, give water every two to three days if it’s dry.

At Homeacres, it’s rare that I water cauliflowers. The main time they need water is in late spring to early summer, if the weather is dry, as the first plantings grow large and then mature.

How much

Cauliflowers grow best in temperate climates with steady moisture. If your climate is dry, and perhaps hot as well, you may struggle to grow them unless you can provide a lot of water.

Extra mulch to retain moisture?

Worthwhile only in very dry climates.

Leaf removal

As the plants grow, their lowest leaves start to yellow and look diseased. This is normal. Remove them by pushing downward on their stems, at the same time as twisting them off.

Old leaves can go on the compost heap. This reduces slug numbers and allows you to spot and pull any weeds.

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Harvest times and methods

How to judge readiness

When plants are large, check every few days for the first signs of a curd forming. Once you see a baby curd, hopefully with lots of healthy small leaves still wrapped tightly around it, you can expect a large cauliflower within about one week, depending on the level of warmth. In a cool spring, and for Romanescos in early winter, this could take a month.

A purple Grafitti cauliflower, now just 65 days since transplanting – it can either be harvested or left for just a few days to grow larger

Thirteen days later, we have an impressive final harvest – these plants were sown less than four months ago

How to pick

Use a sharp and solid knife to cut through the main stem, as well as a few leaves. The leaves are edible and tasty, as with broccoli leaves. It’s another advantage of growing your own, the bonus of more to eat from one planting.

When to pick and how often

Unlike broccoli, cauliflower curds are a single harvest, followed by no, or very little regrowth of any new curds. Romanesco sometimes makes secondary curds, but they are small and rarely worth waiting for in my experience. Secondary growths on widely spaced plants is not efficient, in terms of harvest weight per area.

Summer cauliflowers can quickly open into seeding florets, with some decay and off colours, all within a few days of them having been a beautiful cauliflower. Don’t wait too long in summer before cutting your glittering prize.

Cauliflowers in autumn and spring will stand in good condition for longer, especially in spring when there are fewer insects to eat them. Likewise, when Romanesco are maturing in late autumn, or even early winter, they stand for up to a month because they are maturing so slowly.

Storing

At temperatures below 10 °C/50 °F, cauliflowers may store for up to a week. If they were very mature when you cut them, they are best eaten within two to three days, because some fungal decay and discolouring is probably already happening.

Saving seed

As for cabbage, I suggest leaving this to professionals, because of the space, time and expertise needed. See Real Seeds website for more information.

Potential problems

Which pests are likely, and when

As for cabbage, plants are susceptible to pigeons, rabbits and several insects. Most protection from insects is needed in summer months.

There is nothing worse than caterpillars in otherwise lovely cauliflowers, likely from midsummer and through autumn.

Romanesco cauliflower on 27th July – these have been covered with mesh for three weeks since transplanting

The same Romanesco one month later, showing much stronger leaves with age; we are now spraying every 18-20 days with Bacillus thuringiensis

26th October – there are brown tips on this head of Navona Romanesco, damage from a delay in harvest

Early November – this is an unprotected cauliflower, four months after transplanting; see the significant leaf damage compared with the protected plant in the next photo

A cauliflower from the same planting, pictured on the same day – you can see the huge difference, thanks to this plant being protected by a mesh cover for the first ten weeks after transplanting

14th April – a strong harvest of these Jerome cauliflowers, from the July sowing

Protections needed

Use a cover of fleece for early plantings to give warmth, and cooler mesh for later plantings to protect from insects. If using mesh on spring plantings, use the finest grade in order to reduce damage from flea beetles, which are worse in that season compared to summer and autumn.

Fleece can even sit directly on transplants, while mesh is usually heavier and is better supported by hoops when plants are small.
The most damage from flea beetles is to the small leaves at the centre of plants, because some plants then struggle to even grow at all, after losing their growing tips.
When flea beetles eat leaves through a mesh cover, the damage is not too significant, hence it can work to drape mesh on top of leaves.

Keep a cover on young plants, until you feel confident that insects and pigeons are not going to eat them. This may mean a cover until maturity!

For protection from caterpillars on uncovered plants, spray Bacillus thuringiensis every 20 days.

The finest grade of mesh protecting some newly transplanted cauliflowers at the end of April; for some reason this mesh is very light so I didn’t use any hoops

Mid-May – you can see flea beetles on top of the mesh, managing to eat leaves a little through it, but far less than if plants were unprotected

The mesh has done its job! Now, on 19th June, the cauliflowers are pest-free and one is ready for the first harvest

Other likely difficulties

Clubroot

This is possible but relatively rare. Spores are most active in warm soil, with less damage to overwintered cauliflower plants.

Clubroot results in galls or swellings, on roots and lower stems. These impede the uptake of nutrients and water.

It is spread through any import of infected plants, or from infected neighbouring plants, by use of infected tools, moving infected soil and even from surface water runoff. Spores remain dormant for several years, until coming to life again on host plants.

You may discover resistant varieties for sale – worth a try but no guarantee. Clapton F1 is sold as ‘clubroot resistant’.
Clubroot thrives in slightly acid and wet soils, while no dig reduces the incidence. Worth spreading a little lime in spring every year to lessen the damage.

Non-heading of summer-sown cauliflowers

Caulliflowers which you expect to head in the autumn but do not, suggest that in the following year you need to sow two to three weeks earlier.

And finally

Clear

Simply twist each stem firmly, to remove it with a few large roots, leaving the rest in the bed. In dry weather, I find it’s worth walking on the bed after any such removal, when it has caused unwanted loosening of soil.

A summer harvest of calabrese and Sunset cauliflower, while the Cabbice cabbage are still growing in the soil – these plants were all transplanted at the same time, with 40 cm/16 in spacing, although you see they all finish at slightly different time

Interplanted leeks with cauliflowers in June – the cauliflowers will mature in a month, whilst the leeks will start growing slowly, before accelerating later

12th July – in summertime the garden is changing quickly; top left are new dwarf bean plants, transplanted today after cauliflower; you can also see courgettes, celery, and a new planting of broccoli, made after we had cleared broad beans

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Follow with

Usually one clears ground completely before the next planting. See the photo above showing how we experimented with planting leeks between cauliflowers. This did work, although the leeks struggled for their first month until we cleared the cauliflower plants, and they took time to grow strongly.

Summer harvests can be followed by French beans, salad onions, chard, beetroot and many salads.

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Kohlrabi

Brassica oleracea, Gongylodes Group

I apologise, to German readers especially, for giving kohlrabi a small, but sweet, mention. It's an amazing vegetable, with the swelling of its stem just above soil level being the part that we value and eat. It is sometimes mistakenly considered a root vegetable.

Kohlrabi is of the Brassica family, and susceptible to the same pests. It is strictly an annual, often maturing in half a year, and keen to flower in springtime, according to variety.

Kohlrabi is sometimes compared to turnip. This is totally unfair because the flavour is way more agreeable, the texture is firmer, and it's tasty both cooked and in salad. The main thing to beware of is woodiness, which happens only in early summer, the time when flowering initiates.

Harvests in temperate climates are best either before midsummer, from sowing very early, or in late autumn through winter from summer sowings.

The first, early sowing date can provide about six weeks of harvest before midsummer.
Summer sowings can give six months of harvest, plus the possibility of storage in autumn and winter.
Kohlrabi are beautiful plants, floating their swelling stems above the ground.

Azur Star kohlrabi on 17th May, now three months old and about two weeks from harvest; they had fleece over until early May

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Early Purple kohlrabi in late May, sown under cover in mid-February and transplanted in mid-March, then with fleece over; a first harvest could be taken now

Late November – a 15.9 kg/35 lb harvest of Superschmeltz kohlrabi is ready for storage; these came from 15 plants, sown in early July and transplanted in late July

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Raise plants as for cauliflower, and transplant at 30 cm/12 in.

Pests are the same as for cauliflower. A fleece cover helps spring plantings to grow in warmth, and keeps off many insects. Summer plantings benefit from a mesh cover for their first month.

See the average growth of 19-day-old kohlrabi in late July

These kohlrabi have been planted in late July after clearing lettuce – here you see them without any protection after five days; some were then eaten, so I covered them

Late July – these Superschmeltz and Blaril kohlrabi have been transplanted after a crop of onions

12th August – see the dig (furthest away) and no dig trial beds, growing kohlrabi in the middle, now 17 days since transplanting; in addition to this we have, from left, French beans, cucumber, carrots, kale, celery, beetroot, leeks, chicory and celeriac

By 12th October you see how the kohlrabi has grown, with Superschmeltz in front and Blaril behind; the no dig bed is on the right

Kossak F1 kohlrabi on the right, with vegetables to store in late November

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Kohlrabi are frost-hardy, although I don't know the lowest temperature possible. Mine have survived to -6 °C/21 °F, the coldest it usually gets here in winter. I leave them in the garden, but they also store well in a crate, from harvesting in early winter.

A problem can be splitting across the top, which is maddening in a first sowing when a proportion of the fine kohlrabi suddenly open up. There is still an edible harvest, just less of it, and this seems to follow periods of uneven soil moisture. Keep them well-watered in spring.

January – kohlrabi, at the front, are ready any time in winter, and the cauliflower behind will make curds in spring

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October – a split kohlrabi, Kossak F1, sown in July; most of it is still edible!

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