Growing Fennel – How to Plant and Grow Fennel from Seed

Fennel is a wonderfully aromatic herb with a distinct anise scent and flavor. Depending on which type you grow, it can also be harvested as a vegetable that is delicious roasted, cooked in soup, or added to raw dishes.

Fennel is easy to grow and prefers cool weather. It will draw in beneficial pollinators, including butterflies, if you allow it to flower. The flowers are pretty and ornamental – it’s worth letting them flower to see the blooms and butterflies. There are many things to love about this versatile herb!

Here’s everything you need to know about how to grow fennel, including plant care and harvesting tips. This post focuses on growing fennel. For more detailed information about picking your plants, make sure to stop by this guide to harvesting fennel.

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All About Fennel

Fennel (Foeniculum vulgare) belongs to the carrot family. It’s closely related to other herbs you’re probably familiar with like dill, cilantro, and parsley as well as vegetables like carrots, celery, and parsnips (source). 

There are two main types of fennel: herb fennel and bulb fennel.

Herb fennel (also called common fennel) is grown for the feathery leaves that give an anise flavor to food and are also used in herbal medicine. The seeds that form later in the season can be harvested as well and are used by both chefs and herbalists.

Bulb fennel (known officially as Florence fennel) is grown for the swollen stem that forms right above the ground. When fully formed, these bulbs are harvested and can be cooked or eaten raw.

Both types of fennel are usually grown as annuals, although they are technically short-lived perennials (meaning the plants fade out after a few years). They thrive in cool weather, which is important to keep in mind as you plan your planting schedule.

On average, herb fennel will grow about 3-5’ tall! This means that just one or two plants will provide you with a lot of foliage.

Bulb fennel is typically shorter – about 2-3’ in height. Each plant will produce one bulb, so grow as many as you think you’ll eat.

Best Fennel Varieties

When picking out fennel seeds or plants, the most important part is choosing either bulb fennel or herb fennel, depending on which one you want to grow. Here are some of the top cultivars in both categories. Some varieties are more difficult to find seeds for. I’ve linked to seeds when possible to help you out.

• ‘Florence’– Green-leaved variety grown for its bulbs and foliage. This is the most common fennel variety and one of the easiest to grow. If you’ve never grown fennel before, it’s a great choice.
• ‘Grosfruchtiger’ – Vigorous green fennel (herb)
• ‘Bronze’ – Bronze-leaved fennel with a reddish hue that is edible and ornamental (herb)
• ‘Orion’ – Bolt resistant hybrid with large bulbs (bulb)
• ‘Orazio’– High yields of round bulbs (bulb)
• ‘Solaris’– Fast-growing, semi-flat bulbs (bulb)
• ‘Fino’– Very reliable, best for fall harvest (bulb)

How to Grow Fennel from Seed

Outdoor Sowing

Like other plants in the carrot family, fennel develops a long taproot, which means that transplanting frequently isn’t successful. For this reason, the preferred method is to plant your fennel seeds straight in the garden unless you need to get a headstart indoors.

You can start sowing seeds any time after the last frost in spring. It’s good to get them in the ground as soon as possible because the plants grow best in cool weather.

Seeds should be planted about ¼-½” deep and spaced a few inches apart to start with. Once your seeds germinate and grow a few inches tall, thin them to a spacing of at least 12”.

Germination usually takes 7-14 days in an optimal soil temperature of 60-70°F. If you planted in colder soil, it may take a little longer. 

Sowing Fennel Indoors

If you only have a short period of cool weather in the spring, you may want to start seeds indoors so that plants will mature more quickly when they go in the garden.

You can grow fennel in seed-starting trays, but biodegradable pots are a better choice. This lessens the transplant shock when you transition the seedlings to your garden, which is important because fennel doesn’t always transplant well.

Start your seeds indoors about 4-6 weeks before your last average frost date in the spring. Sow them ¼-½” deep and keep the soil damp while they germinate. Provide your seedlings with plenty of light once they sprout and water them before the soil dries out.

Be sure to harden off the seedlings about 1-2 weeks before planting them in your garden. Discover how to prepare your seedlings for transplanting.

Carefully rip open the bottom of your degradable pot before transplanting. Many pots take a long time to degrade, which can impede your plant’s growth.

Fennel Planting Tips

When to Plant Fennel

Seedlings that you bought or started indoors can be planted in the garden after all danger of frost has passed. However, if you’re growing bulb fennel, there’s another option you may want to consider: fall planting.

Because fennel likes cool weather, plants are more likely to produce small bulbs if planted in the spring when the weather goes from cool to hot. If planted at the end of summer and allowed to grow into the cool weather of fall, the bulbs typically get much bigger and suffer from fewer pests.

To get a fall harvest, start seeds indoors for better germination and plant seedlings at least 8-10 weeks before your first frost date in the fall.

Fennel Growing Conditions

Getting the growing conditions right is important when it comes to how to grow fennel successfully.

To start with, fennel needs a location that gets full sun. Plants can take a little shade once the weather gets hot, but too much of it will cause them to grow poorly and flop over.

The soil should be well-drained and preferably enriched with compost. This is especially true for bulb fennel because it needs a good amount of nutrients to form large, flavorful bulbs.

Worm castings are another good way to add natural, slow-release fertilizer to your soil. I think they’re a safer bet than purchased compost. Unfortunately, gardeners have lost plants due to purchased manure and compost that were contaminated with herbicides. For a more lengthy discussion of this, and how to feed your garden affordably and safely, be sure to read Compost Everything.

An interesting aspect of fennel is that it produces compounds from its roots that inhibit the growth of most other garden vegetables. For this reason, it’s best to give fennel its own space in your garden where it won’t affect any other plants.

Also, avoid planting fennel where any members of the carrot family have been in the last few years. 

You can grow fennel in containers, as long as they are large enough. Use a 7-10 gallon container to keep your plant happy. Seven gallon grow bags are a good choice. The nonwoven fabric breaths well, which keeps soil from becoming waterlogged. The flip side is that you will probably have to water more frequently in order to keep your fennel bulbing nicely.

Adding gravel to the bottom of the container for drainage purposes is frequently recommended. Don’t do this. Adding gravel to growing containers is a gardening myth that won’t go away. It can actually have the opposite of the intended effect and create a layer of soggy soil. For a detailed, scientific explanation of why gravel in containers does not improve drainage, check out this post from Deep Green Permaculture.

Finally, be sure to give your plants plenty of room. They should be spaced at least 12” apart to allow for good airflow. If you plan to leave herb fennel plants in your garden for a few years, they may eventually grow up to 5’ tall, so plant where they won’t shade anything else out. Some gardeners decide to make a simple fence or trellis to contain large herb fennel plants:

Caring for Your Fennel Plants

Fennel plant care is pretty easy, since this is a very low maintenance herb.

Once the weather starts to get warm, you can apply a light layer of mulch around your plants to help keep soil moisture in and weeds down. Try to make sure your plants get consistent water (but not soggy soil), especially bulb fennel.

Herb fennel doesn’t need much in the way of fertilizer, but you can apply a layer of compost around plants every year if you are growing them as a perennial.

Bulb fennel may not need fertilizer if you added compost to start with, but an application of a potash fertilizer every 2-3 weeks can give a boost to bulb formation. One study showed that an application of nitrogen and potassium fertilizers lead to “higher vitamin C, total phenols and flavonoid contents, and antioxidant capacity” in bulb fennels (source). Potash fertilizer is high potassium. Organic nitrogen sources include blood meal and feather meal.

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If you are growing herb fennel, you can either let your plants flower and form seeds (which are edible), or you can clip the flower heads off as soon as they appear to prolong the leaf harvest.

We love using fennel seeds to make pasta sauce with our homegrown tomatoes. Try letting some of your plants flower so you can collect the seeds!

Blanching Fennel Bulbs

There’s one final, but optional, step to take with bulb fennel. It’s a technique known as blanching, which should not be confused with the process of boiling and cooling vegetables that goes by the same name.

Blanching keeps the fennel bulbs white and tender by protecting them from sunlight.

All you need to do is mound soil over the bulbs as soon as you see them start to form. Do this a few more times throughout the season as they grow and push their way out of the soil.

Fennel Pests and Problems

Now that you know how to grow fennel, what do you need to do to protect your crop from pests?

Fortunately, not many pests bother fennel because of its strong smell. Deer and other critters usually stay away, although it’s still worth putting up some protection because hungry herbivores will take a bite out of almost any plant. As Rhonda Massingham Hart says in Deerproofing your Yard and Garden, a hungry deer will eat anything, including the paper this list of deer-resistant plants is printed on. 

Slugs and snails can do damage to seedlings when the weather is cool and damp. Try putting crushed eggshells around your plants and avoid laying mulch down until the weather dries out. Homemade slug traps can also help. 

One “pest” you might see on your plants later in the season is the swallowtail butterfly caterpillar (also known as the parsley worm).

You can recognize these caterpillars by their distinct green and black stripes with yellow spots. They will feed on the leaves of fennel plants before eventually forming a chrysalis. It’s so much fun to watch your “parsley pets” (what our daughter calls them) grow and change each day. 

Because the swallowtail butterfly is an important native pollinator, these caterpillars are a beneficial insect, even though they may seem like a pest. Let them be or transfer them to another member of the carrot family if you want to prevent damage to your fennel. Because fennel plants are larger than many other umbellifers, they can handle the caterpillar damage better. I prefer to have them on fennel than other, shorter-lived relatives like cilantro. 

One of the most common problems with growing fennel is that fennel doesn’t always bulb well. If you want bulbs, make sure to select a bulb variety. Provide ample, consistent water and pick a variety suited to your climate. Make sure to get a bolt-resistant variety if you live in the South and usually have hot and/or short springs. If you live in the South, you may want to experiment with growing fennel in the fall instead of the spring.

Harvesting & Storage

Herb fennel is easy to harvest. You can start clipping off sprigs as needed when the plants get at least 8-12” tall, but try not to harvest more than ⅓ of the plant at any given time.

Later in the season, you can also harvest fennel seeds when they ripen and turn brown. Simply clip off the dried seed heads and shake them out over a container. Then, lay them in a dark, dry area to dry for about 2 weeks before storing them.

Bulb fennel is usually ready to harvest about 70-80 days after planting. You can harvest early on for baby bulbs or wait until they get about the size of a tennis ball.

Cut the bulbs off right above soil level, leaving the root in the ground. The roots may resprout and send up shoots, which you can cut off and eat.

To store fennel bulbs, cut off the foliage and all but a few inches of the stem. Store them in the refrigerator where they will keep for several weeks.

Enjoying Fresh Fennel

Knowing how to grow fennel means that you can enjoy both the herb and bulb form of this very unique plant. Both thrive under similar conditions, so you can grow them next to each other in your garden.

If you ever get tired of harvesting, let your plants bloom to draw in local pollinators and make them very happy!