Top of The Crops - Fennel

Welcome to Top of the Crops - today, you will learn how to grow fennel in a polytunnel. For more gardening insights, be sure to check out our blog Polytunnel Gardening too!

Growing Fennel In A Polytunnel

Fennel, Foenicium vulgare, has different though closely related species, one commonly used as a vegetable, the other a perennial herb. Both of these related plants might be grown in a polytunnel garden

Herb fennel can be a useful herb for salads and garnishes, and Florence fennel pairs well with fish and can be used in a wide range of recipes as a versatile vegetable. 

The Preferred Conditions for Fennel

Herb fennel, also sometimes referred to as a common fennel, is hardy, drought tolerant and easy to grow. It will do best in full sun and free-draining soil.

One important thing to note if you wish to grow herb fennel in your polytunnel or elsewhere in your garden is that it should be sown apart from many other crops, as it can have a detrimental effect on their growth. 

Growing it in a container, therefore, could be a good solution. It will grow well with dill, which could be sown alongside it, though the crops cross-pollinate so this should be avoided if you wish to collect seed to sow the following year. 

Fennel used as a vegetable, Florence fennel as it is often called, also likes full sun. However, it will do best during warm, damp summers and will need consistent moisture to do well. A consistently moist but reasonably free-draining soil is required for best results. 

This is much more challenging to grow than herb fennel as it can be fussy, and quick to bolt if the conditions are not quite right. 

Month By Month

Herb fennel:

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Florence Fennel:

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Sowing Fennel

Herb fennel is sown from March and Florence fennel from April and both types can be sown up until July. You can either sow indoors or outside/ in a polytunnel where the plants are to grow. 

However, it is important to note that both plants can be fussy about root disturbance and so whichever option you choose, you need to make sure you keep this to a minimum. 

Sowing Indoors

If you decide to sow fennel of either type indoors, you should sow the seeds individually into modular trays, soil blocks or small biodegradable containers. This will help ensure that root disturbance is kept to a minimum when you later transfer young plants outdoors. Sow seeds around 1cm deep. 

Sowing outdoors

If you direct sow where the fennel is to grow, again, sow seeds around 1cm deep, or perhaps a little deeper with Florence fennel. Sowing in drills of this depth is a popular option. The drill is created and watered before the seeds are sown thinly along the rows. If more than one row is created, these should be around 30cm apart. 


Seedlings should usually be thinned to leave a spacing of 30cm between plants for both types of fennel. But if you are growing Florence fennel for baby bulbs then you can space plants at 10-15cm apart and harvest when the stem or 'bulb' of the plant is around 5cm across. 

Transplanting Fennel

Herb Fennel sown indoors should be planted out from May onwards, Florence fennel can be transplanted in April or May, though you should take care not to plant out too early and should wait until after the last frosts where you live. Remember that you will need to harden off young seedlings before placing them outdoors. 

Fennel can also be planted out (or sown directly) into large containers filled with multi-purpose potting mix. As a general rule of thumb you can sow or plant three Florence fennel plants into a container around 40cm wide or one herb fennel plant per 30cm pot. 

Care Tips for Fennel

Herb fennel is generally a fairly easy and straightforward plant to grow, and it will require little ongoing care when placed in the right position. Florence fennel, as mentioned above, can be a bit more fussy, and so you do need to ensure that you not only place it in the right location but also provide the right care. 


Herb fennel is tolerant of dry conditions and will typically not need watering regularly when growing outdoors. But you should keep the plant well-watered for the first few months of its growth to get it off to a good start, and of course will need to water when growing undercover in a polytunnel. 

Florence fennel, on the other hand, is much fussier and will struggle if allowed to dry out, or left in waterlogged conditions. So you need to make sure you water 'just right' and keep things in the 'Goldilocks zone'. 

Make sure you provide consistent moisture through the growing season but also always ensure that excess water can drain away reasonably freely. Dry conditions and waterlogged conditions can cause stress, which can cause the plants to bolt. 


Weed around your plants to reduce unnecessary competition for water and nutrients in order to achieve the best results. It is best to weed by hand to avoid any damage to the plants. Remember also that mulch can help suppress weed growth, so add an organic mulch around your plants. 


If you are growing herb fennel in a windy location, then it may be beneficial to provide a cane, stake or other form of support to prevent breakages of the tall flower stalks. 

Cutting Back

Herb fennel is a perennial plant that will return in your garden over multiple years. It can also self-seed extremely readily, springing up all over your garden if you give it the opportunity to disperse its seeds. 

If you do not want the fennel to self-seed then you should make sure that you remove faded flowers or immature seed heads before these fully mature. 

However, if you wished to harvest fennel leaves only, and not the flowers or seeds, then you may already have cut off the flowering stems down to their bases earlier, to encourage more foliage growth. 

It is also worth remembering that fennel should be left to set seed in a wildlife-friendly garden because it is a good food source for a number of birds and other wildlife. 

You can if you wish simply leave the fennel stems and leaves to die back naturally at the end of the growing season. New ones should then spring up from the base in the spring. The dead hollow stems are great for insects in winter, and can also be an attractive architectural feature in a winter garden. 


Fennel is propagated by seed, which can be collected in late summer or autumn or left to self-sow if desired. Since fennel does not like root disturbance, you need to be careful, but self-sown fennel seedlings can be lifted and transplanted elsewhere. This is generally easier and more successful than trying to move some or all of a mature, established fennel clump. 

Harvesting Fennel

With herb fennel, you can harvest sprigs of young leaves as required throughout the latter part of spring, summer and into autumn. Harvesting regularly will promote further foliage growth. You can also harvest and eat the flowers in salads, and also collect the seeds to use fresh or dried. 

Florence fennel is ready to harvest, depending on when it was sowed, in the late summer or early autumn. The swollen stems or 'bulbs' as they are commonly but incorrectly called, are usually harvested when they are around 10-15cm across,  but you can also harvest them at any size and they can be taken as baby bulbs when just 5cm across. 

With Florence fennel, you need to harvest before the plant flowers, as at this point the stems become woody and inedible. 

You can harvest by either uprooting the whole plant or by slicing it off just above the base leaving the root section in the ground. If you take the latter option then the section still in the ground can regrow small leafy shoots that are an additional harvest and which can be used in salads. 

Once you have harvested fennel you will find that there are plenty of recipes to use fennel that you have grown. 

Storing Florence Fennel

Florence fennel is best used fresh from the garden, as soon after harvesting as possible. But it can also be stored in your fridge or another cool and dry location for a few weeks. 

Varieties of Fennel

Varieties of herb fennel include:

  • Green fennel – the common fennel type, Foeniculum vulgare. 

  • Bronze fennel/ purple fennel – a purplish or bronzish variant,  Foeniculum vulgare 'Purpureum'.

  • Wild fennel – the wild perennial form, Foeniculum sativum. 

Varieties of Florence fennel include:

  • Dragon

  • Fennel de Firenze

  • Rondo

  • Sirio

  • Zefa Fino

Common Problems for Fennel

Fennels do not usually have a huge number of pest problems, and can in fact sometimes help to repel, confuse or distract pests of other crops. But look out for slugs and snails when growing Florence fennel. 

Most commonly, however, problems with fennel, especially Florence fennel, do not relate to pests or diseases but rather to issues with the environmental conditions or care. As mentioned above, Florence fennel can be prone to bolting when things are not quite right. 

Top Tips for Growing Fennel in a Polytunnel

Both herb fennel and Florence fennel can potentially be useful additions to a polytunnel garden. But  you will have to give careful thought to where precisely they are placed so that you do not end up with excessive competition or 'bad companions' being too close to one another. 

Watering is another important thing that you need to get right when growing fennel, especially Florence fennel, in your polytunnel. Set up a good quality irrigation system so that you can easily create the consistently moist but not waterlogged conditions that these plants require. 


Does Fennel Come Back Every Year?
How Do You Grow Fennel UK?
What Not To Plant Next To Fennel?
How Long Does It Take To Grow Fennel?


Francis, A., (2023) 47 Fennel Recipes for Shaving, Roasting, and Then Some. Bon Appetit. [online] Available at:

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growing fennel in a polytunnel