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Bulbous fennel (Foeniculum vulgare) is a cultivated form of wild fennel, which originally comes from the Near East and the Mediterranean. The plant reaches a height of 50 to 150 cm and forms a bulb in the lower area above the ground, which is also colloquially called a tuber. This is formed from the fleshy, thickened leaf sheaths of the finely feathered leaves. But what do you have to consider when growing fennel and when is it ripe?


Fennel needs a sheltered location where it is as warm as possible. Beds on the south or south-west side of a house are therefore optimal. With growth heights of more than one meter, the bulbous fennel is one of the large perennials that require a relatively large amount of space.

  • Light requirements: sunny to light semi-shade
  • warm but protected from the heat
  • shaded if possible in the midday hours


Foeniculum vulgare is not particularly picky and will grow in almost any regular garden soil. The soil doesn't even have to be particularly fertile. However, the tuber fennel thrives best in loose and deep soil, as the plant develops a long taproot.

  • relaxed
  • profound
  • moderately dry to moist
  • Clay or loess soil
  • pH value: neutral to slightly calcareous (6.5 to 7.5)
  • conditionally lime tolerant


Before you start selecting fennel seeds, there are a few important characteristics to consider. In general, a distinction is made between types of fennel that are resistant to bolting and those that are not. In our climate, it is advisable to only use bolt-resistant varieties. In addition, the numerous fennel varieties are differentiated according to their harvest time. For example, 'Finale' and 'Rondo (F1)' are suitable for early cultivation, corresponding autumn fennel varieties are 'Rudy' or 'Sirio'.


Since fennel is sensitive to cold, it is advisable to grow the seeds of early varieties in a greenhouse or in small pots on the windowsill from the beginning of April.

  • Substrate: potting soil, cactus soil
  • moisten slightly
  • Sowing depth: 1 cm
  • Protect pots from evaporation with foil or a freezer bag
  • Germination time: about 3 weeks
  • Germination temperature: 20 to 25 degrees


After a few weeks, small seedlings will have developed from the seeds. After germination, place the pots or seed trays in a bright but slightly cooler position. A location in a little heated, bright room at around 15 to 16 degrees is best. As soon as the first true leaf appears after the cotyledons, the seedlings are transplanted into larger pots. Be very careful when doing this, because the fine root is still very sensitive.

  • Temperature: not above 16 degrees
  • bright, without direct sun
  • preferably use slender, tall vessels
  • Keep substrate slightly moist


After another five weeks of cool and bright rearing, the young plants can move into their final location in the bed or greenhouse from mid-May. It makes sense to plant green manure in the autumn of the previous year to enrich the soil with nutrients. Fertilization is also possible shortly before planting out with mature compost.

  • Planting distance: 40 cm
  • Row spacing: 35 to 40 cm
  • plants with 3 to 4 leaves grow best
  • Leave the tuber base above ground level

In order to protect the young plants, which are sensitive to the cold, they should be covered with a fleece or a foil tunnel until the temperatures rise permanently above 12 degrees.

direct sowing

Due to the relatively short cultivation time, autumn fennel varieties can also be sown directly outdoors, in a raised bed or cold frame. In this case, sowing takes place in rows between mid-May and the end of June. Protect the seeds and seedlings from cool temperatures with fleece or a polytunnel until the end of May.

  • Time: from mid-June outdoors
  • from May in the cold or raised bed
  • Row spacing: 35 cm
  • separate after germination
  • Distance: 40cm

Mixed culture and crop rotation

As with most vegetables, the neighbors have a great influence on the thriving of the plant. The right combinations can support sweet fennel growth and even keep pests away. On the other hand, unfavorable neighbors may give off plant substances that prevent the fennel from developing well.

  • good neighbors: Cucumbers, lettuce, cauliflower, radishes, tomatoes
  • bad neighbors: other umbellifers such as carrots, parsnips, parsley, celery

Fennel may only be grown again in the same bed after three years at the earliest. In addition, direct crop rotation with other umbellifers should also be avoided.

Planting in the bucket

In general, fennel is grown in the garden bed or in a raised bed. But it also grows easily in plant pots or tubs on the balcony. However, a sufficiently large planter and suitable substrate are important for cultivation on the balcony. If the planter is too shallow, tuber formation will be disrupted.

  • Planter: at least 10 l volume
  • choose high forms (forms long taproots)
  • must include drainage hole
  • Create drainage from lava granulate, chippings or expanded clay
  • Substrate: loamy citrus plant soil, humic vegetable soil

The care of the fennel on the balcony differs little from that in the field. Tub plants only require a little more care and attention when watering.


In addition to the correct watering behavior, it is important that the soil in the bed always remains finely crumbly and loose. Therefore, regularly clear the bed of weeds and carefully loosen the soil with a hoe. Otherwise, the tuber fennel is one of the less demanding vegetable plants and is not particularly labor-intensive to cultivate.


Fennel prefers moist, fresh soil. The plant does not tolerate waterlogging and prolonged drought. Sufficient moisture and possibly mulching the soil with straw or grass clippings prevent the sweet fennel from starting to shoot in summer when it is hot and dry. The more evenly moist the plants are kept, the larger the tuber can develop.


If at all, fertilize the fennel only moderately at the beginning of the growing season. Poor soil can be enriched with some compost or horn shavings before planting. The plant tends to store high levels of nitrate and produce excessive foliage when nutrient levels are too high.

pile up

So that the fennel forms a particularly tender, snow-white tuber, the plants should be earthed up about two weeks before harvest, i.e. ten weeks after sowing. To do this, carefully pull the loose soil towards the tuber with a hoe without damaging the roots. The lower, light-colored areas of the leaf stalks should be just covered with soil, leaving the green part exposed.

To harvest

The cultivation period of fennel is about twelve weeks between sowing and harvesting. But when is it ripe? You can harvest the fennel when the bulbs are about the size of a fist. This means that plants grown early on the windowsill in April can be harvested in July/August. Plants sown outdoors usually mature by September. Once the fennel has ripened, it should not be left for too long. Because otherwise it loses a lot of quality, especially in the heat. The plants change their metabolism and begin to grow vigorously, resulting in lignification and eventual tuber bursting. If you leave the roots in the ground after harvesting, they will produce new shoots that can be used in soups or salads.

To store

Before storing, cut off the green shoots from the tuber. You can store the fennel in the vegetable compartment of the refrigerator for up to three weeks. It is best to wrap the fennel bulb in a damp cloth. Alternatively, it is possible to store them in a cool, frost-free place such as the garage or an unheated basement. You should keep the tubers in this case in moist sand.


When the fennel flowers, elongated seeds form after a short time. These can be harvested and either used as a spice or sown in the coming year.


diseases and pests

Snails like to attack the young plants. Good snail prevention is therefore very important. Occasionally, the plants are also attacked by aphids. Always avoid using pesticides on vegetable plants. It is better to use proven home remedies or biological alternatives such as parasitic wasps. In unfavorable weather conditions, diseases such as fennel rust or downy mildew can occasionally occur in the tuber fennel.

Common problems

The most common problems when growing fennel bulbs include two closely related phenomena: the plant does not form a tuber or the fennel has budded. To avoid these problems, consider the following tips.

  • the fennel does not form a tuber:

Make sure to use bolt-resistant varieties, especially when sowing early. In order for the largest possible white tubers to form, flowering must be prevented at all costs. There are many reasons why the fennel bulb remains small.

Too high temperatures after germination

Plants planted early run the risk of growing too quickly. Especially if the temperature is too high after germination. Cultivation of the young plants at temperatures no higher than 16 degrees is optimal. In addition, the young plants must not be planted too deep, otherwise no large tuber will form.

Insufficient planting distance

Foeniculum vulgare needs a lot of space. If the plants are too close together, they deprive each other of nutrients, light and water, so that long leaves tend to form in the competition. Make sure each plant has at least 35cm of space in all directions. During the growth phase, regularly loosen the soil and remove the weeds.

heat and drought

If you have delayed sowing or planting later fennel varieties for too long, only long, flat tubers will develop from August. Especially when the temperatures are high and the soil is too dry. Young plants with about four leaves grow best. Make sure the soil is moist and water regularly. Both mulching and partial shading protect against excessive evaporation.

  • the plant has shot:

One of the biggest problems with cultivating Foeniculum vulgare is its tendency to bolt. In the case of fennel, the formation of flowers is triggered by so-called long-day conditions. This means the days of the year that bring more than 14 hours of daylight. High temperatures accelerate this process. In addition, the tendency to bloom is promoted by stress. Such adverse conditions include the following.

  • dryness
  • heat
  • >14 hours of daylight
  • strong temperature fluctuations
  • cold

Tips to avoid

  • preferably plant bolt-resistant varieties
  • do not sow until June
  • Keep soil evenly moist
  • possibly install a shade
  • mulch soil
  • the tubers are not yet ripe in autumn:

Although Foeniculum vulgare tolerates light frosts, the quality of the tuber suffers significantly from strong temperature fluctuations. Harvest time can be extended by a few weeks under a polytunnel or a double layer of fleece. By November at the latest, when the temperatures drop below zero for a long time, the fennel bulbs have to be removed from the bed.

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