Fennel can be started from seed indoors in pots or planted directly outdoors when there is no longer any risk of frost. Fennel prefers a slightly acidic (pH 5,5-6,5), fertile and well-draining soil.
Fennel is best started from seed. To propagate from root cuttings or plant division is more difficult as the fennel plant, much like dill and cilantro (coriander), develops a long and delicate taproot that does not respond well to handling.
We grow fennel at home and it is one of our favourites. As an aromatic herb, fennel offers an easy to recognize anise-like flavour and scent. And much like for example horseradish and cilantro (coriander) its strong flavour profile ensures that a little goes a long way.
For us, fennel is both a garden vegetable and an aromatic herb. We use the fine leaves and root bulb as a vegetable while the dried seeds are a loved herb to be used in both cooking, teas and baking.
Name: Fennel (Foeniculum Vulgare)
Height: 1,5-2,1 metres (4-7 feet)
Soil: Prefers well draining, fertile and slightly acidic soil
Description: Hardy to half-hardy perennial aromatic herb that is often grown as an annual. Easily confused with dill. The fennel plant is bright green with feathery leaves and clusters of small flowers.
Use: Harvest and use leaves, bulb as well as seeds for cooking, baking and spice mixes.
How to start fennel from seed – proven method
Our proven method for growing fennel starts indoors about 4-6 weeks before the last expected frost.
Young fennel seedlings and plants do not tolerate frost and freezing temperatures. Starting indoors ensures an early start protected from surprise weather and gives us an earlier harvest.
1. Choose your starter pot
Use a seed starter tray with a covering plastic dome or regular starter pots that you cover with plastic.
And as always, all pots need drainage holes.
2. Use a slightly acidic soil
Fennel prefers a slightly acidic potting soil mix. It is important to choose a potting soil mix that drains well.
3. Start seeds in groups of 3 – 5 seeds per pot
Plant your seeds in groups of 3-5 seeds and approximately 1 cm (½ inch) deep. Take care to place the seeds close to each other, not on top of each other.
I usually sprinkle the fennel seeds across the top and then cover with soil. Later I will snip or cut unwanted seedlings keeping only the two strongest in every pot.
You can of course plant 1 seed per pot if you know your seeds to have a 100% germination rate. But from experience, I know that a 70% germination rate is good to normal why I always plant fennel seeds in groups.
Also, it is not guaranteed that all seeds that do germinate will produce a strong seedling. So, play it safe and plant a few more seeds than the number of plants you are planning to grow.
To help your seeds germinate, keep the growing environment moist by covering it with plastic. We use seed starter trays with plastic domes. But you can just as well use a starter pot and cover it with plastic film.
You can also pre germinate your fennel seeds using the paper towel method. This way you make sure you are only using strong and viable seeds.
4. Remove cover as seeds sprout first leaves
Expect first leaves or cotyledons to sprout in about 7-10 days.
Remove the dome or covering plastic film as the seeds germinate and sprout first leaves. Place the seedings in a location with good light for at least 6 hours per day.
The seedlings need good light to avoid growing leggy. Still you do not want to place the seedlings in a location that gets too hot. Think light, bright and room temperature.
5. Select the strongest seedlings
When starting 3-5 seeds per pot or cell you are bound to have more than 1 seedling per space. At least in some pots or cells.
As the fennel seedling develops a delicate taproot, we do not try to prick out (video tutorial) or divide the seedlings (video tutorial). Instead, snip or cut the weaker seedlings (video tutorial), leaving only the two strongest and dominant seedlings.
Do not try to remove the weaker seedlings by the root as you risk disturbing the root system of your chosen dominant seedling. Cutting the weaker seedlings will ensure that they wither down and die. Read more about How to thin seedlings without killing them.
6. Transplanting fennel seedlings outdoors
Transplant your two dominant seedlings when they are large enough to handle and there is no longer any risk of frost.
Do not attempt to separate the two seedlings growing in the pot. Treat them as one seedling and transplant the whole pot.
You can of course snip all but one seedling and only keep the most dominant. But since I harvest my fennel bulbs early I have found it ideal to grow fennel in pairs. I find larger fennel bulbs less tasty and prefer to grow more smaller bubs.
Whereas fennel does not mind being cool, the young plant will not tolerate frost. This is also the reason we start fennel in pots indoors. We like to get a head start, but do not want to risk being caught by a late onset of frost or snow.
Plant seedlings 30 cm / 12 inches apart and allow 40 cm / 16 inches between rows.
7. Mulch, “earth up” bulb and water a lot
Mulch your garden bed with straw to help with soil moisture while protecting against soil borne diseases and pathogens.
As the fennel bulb grows, add more soil around the bulb to help it grow larger and stronger. But do take care not to get soil in between the forming leaves when you “earth up” your fennel bulbs.
How to harvest fennel
Fennel plants let you harvest fresh fennel greens, fennel bulbs as well as seeds.
You can pick fresh fennel greens continuously as the plant grows and develops. To help the plant grow back, do take care not to harvest more than ⅓ of the fennel greens at any one point of time.
Fennel bulbs take anything from 14-18 weeks to grow and develop. Harvest the bulbs when they measure approximately 7 cm / 3 inches across. If you let the bulbs grow too big they get fibrous and very tough.
Fennel seeds are harvested from the clusters of tiny flowers that are dried. Cut the stems and hang them upside down inside a brown paper bag. As the flowers dry the seeds will fall off when the branches are lightly shaken.
How to store your fennel harvest
Fennel seeds are best stored or preserved dried. Keep them in airtight containers in a dark and warm but not hot place.
Fennel greens can be dried or stored in the freezer in airtight bags.
The fennel bulb can be stored in the freezer in portion sized pieces. Cut or cube the bulb and blanch for 30 seconds in salted water. Next place in ice cold water until cool. Now drain and dry the fennel pieces before freezing in containers or freezer bags.
How to propagate fennel
Fennel is best propagated from seeds. As the fennel plant develops a strong, central tap root it is very difficult to propagate fennel by root cuttings or plant division.
Companion plants for fennel
I am yet to read about a good companion plant for fennel. Still, from my experience, fennel is great at attracting pollinators and I have not found it to hurt my other plants.
It is however commonly accepted that strong aromatic herbs can be helpful in keeping harmful pests and even deer away. 
I do however not grow dill next to fennel as they are closely related and I was told by a trusted gardener that they will cross pollinate.
Known pests and diseases
Most herbs and vegetables I grow have at some point or another been attacked by aphids and fungus gnats. I use Neem oil to keep these and many other pests and diseases at bay.
But fennel is also susceptible to fungal diseases like leaf blight, Downy mildew and Powdery mildew. Aside from aphids, fennel plants can also be attacked by pests as armyworms and cutworms as well as root knot nematodes. 
How to use fennel
Fennel is a versatile aromatic herb with a pleasant anise-like flavour and scent. And it is an interesting plant where the fruit and the seed are one and the same.
Fennel seeds are great for dry rubs and spice mixes but also to use with veal, pork and fatty fish like mackerel, salmon and tuna.
Use the fine leaves for fresh and flavorful salad greens or to flavour dips and dressings.
Another favourite is to use the fennel bulb and thicker stems. Choose to cut into pieces and roast or barbecue for a real treat. Or slice thinly and serve in a salad or place on top of, for example fish in a tin foil bag placed on the barbecue.