Growing Fennel Plant: Detailed Care and Harvesting Guide

Fennel is a unique plant because it can be grown as an herb, a vegetable, or both. Some varieties are also grown as ornamental plants because of their decorative foliage.

Easy to grow, you can use freshly harvested bulbs or leaves to cook with, or use the seeds as a medicinal herb. The plants typically repel pests, which makes them very simple to care for.

Here’s a detailed guide on growing fennel plant, including how to care for and harvest it.

What Is Fennel?

Fennel (Foeniculum vulgare) is a member of the carrot family (Apiaceae) and related to other herbs like dill, anise, and parsley. All parts of the plant (bulbs, leaves, flowers, seeds) are edible and have an anise-like flavor.

There are two main types of fennel grown in the garden: herb fennel (also called leaf or sweet fennel) and Florence fennel.

Herb fennel is grown mainly for its leaves and seeds. Plants are perennial in USDA hardiness zones 4-9 and will grow up to 5 feet tall. The common herb has feathery, green, dill-like foliage, but there are varieties that have bronze or bronze-purple leaves.

Perennial fennel can self-seed and become weedy if you don’t pay attention to it. However, the seeds can be used to cook with or for medicinal purposes, so you can simply harvest them to prevent this from happening.

Fennel can be grown as either a vegetable or an herb and has a long history of use as a medicinal plant. Pollinators also enjoy the flowers and will be drawn to your garden when they bloom.

Florence fennel can be grown for its foliage, but it’s more typically grown for the swollen bulbs that are used for cooking. It’s usually grown as an annual and does best in cool weather.

Annual fennel doesn’t get as big as the perennial kind, often topping out at 3 feet. Though called “bulbs”, the part that gets harvested is actually the bulb-like stem that swells up right above ground level.

Uses in the Landscape

Besides being grown as an herb or vegetable, fennel plant has other important uses in the landscape.

The bronze-leaved varieties of perennial fennel are a good addition to an informal landscape or a meadow-like setting. They can be used in a border and make a good addition to a cottage garden.

Fennel plants are also a food source for the swallowtail caterpillar and are therefore a good choice for a butterfly garden. In fact, you may see some of these friendly caterpillars on your plants at some point during the season.

Perennial fennel is a hardy and low maintenance plant to grow. It’s rarely bothered by pests, and the strong scent and flavor make the plants deer-resistant as well.

The entire fennel plant, including the seeds, has a strong anise-like scent and flavor. This typically keeps away insects and larger pests like deer, making it an easy plant to care for.

In fact, though fennel isn’t grown as much as other herbs, there are lots of reasons to add it to your herb garden or landscape!

Top Fennel Varieties

The biggest choice you have is deciding whether you want to grow herb fennel or Florence fennel. After you’ve decided that, there are some good varieties to choose from in either category.

Herb Fennel Cultivars:

• ‘Grosfruchtiger’– Probably the best and most frequently grown green fennel variety. Plants are very vigorous, and the leaves have great flavor.
• ‘Dulce’– Another green-leaved cultivar grown for the foliage and seeds.
• ‘Rubrum’– A popular variety of bronze fennel. Leaves and seeds of this cultivar are edible, but it can also be grown for ornamental value.
• ‘Purpureum’– Another ornamental or herb garden cultivar with bronze-purple leaves.

Florence Fennel Cultivars:

• ‘Fino’– This variety is slow to bolt, reliable, and has a strong anise flavor. Bulbs are a good size and best planted late summer for a fall harvest.
• ‘Orazio’– This cultivar develops very large and thick round bulbs. Higher yield than other varieties.

Different varieties of fennel will have either rounded or flat-ish bulbs. They typically take 70-80 days to mature, making them a better crop to grow in the fall than in the spring.

• ‘Orion’– This variety has compact plants that still produce good size bulbs. Good for spring or fall harvest.
• ‘Solaris’– Fast-growing and matures before many other varieties. Bulbs are large but semi-flat instead of round.
• ‘Mantovano’– A great-tasting variety that matures quickly and can be harvested as either baby or full-size bulbs.

How to Grow a Fennel Plant from Seed

Both types of fennel can be grown from seed, and this is the most reliable method to use (unless you want to buy plants from a nursery).

You can start them outdoors directly in your garden or pots, or you can give them a headstart indoors.

Starting from Seed Indoors

Start your seeds about 8 weeks before your last average frost date in the spring. Florence fennel can also be planted in late summer for a fall harvest, so you can start seeds 6-8 weeks before your planned planting date.

Fennel is best started from seed indoors to give your plants a chance to get ahead of the pests and warm weather. However, you can also direct sow seeds in your garden or pots.

Here’s what you’ll need:

• Fennel seeds
• Seed starting trays
• Seed starting medium
• Plastic covers (optional)
• Grow lights (optional but preferred)
• Water

Before sowing your seeds, mix your seed starting medium with enough water to get it damp. It should clump together when you squeeze a handful, but it shouldn’t be dripping wet.

Then, fill up your trays with the dampened mix. Level off the top with your hand, and make sure there aren’t any air pockets in the soil.

If you are planting bulb fennel, remember that one plant will produce one bulb, so plan accordingly based on how much you want to harvest. Always plant more than you need to make up for seeds or plants that aren’t successful.

If you are planting herb fennel, 2-3 full size plants will produce more than enough foliage and seeds for a whole family. Unless you want to harvest very large amounts, stick with just a few plants.

Plant your seeds ¼ inch deep into your trays. Make sure they are covered with a light layer of soil and water them in well.

If you have plastic covers, put them on your trays now to help keep the soil mix moist while the seeds germinate. Place the trays somewhere warm and out of the way. The seeds should germinate in 8-12 days.

Once your seeds sprout, remove the plastic covers and place the trays under grow lights or by a very sunny window. Keep your seedlings water, but don’t oversaturate the soil.

Don’t forget the important step of hardening off your seedlings before transplanting them! You need to gradually accustom them to conditions outside before putting them in the ground.

Harden off your plants 1-2 weeks before transplanting them by gradually taking them outside for longer periods each day. They are now ready to go in your garden!

Starting from Seed Outdoors

If you want to start your seeds directly in the garden, wait until 2-3 weeks before your last frost date in spring.

Before planting them, make sure you prepare an area in your garden by weeding and removing debris. Mix in a good amount of compost or well-rotted manure, especially if you are planting perennial fennel.

Sow your seeds shallowly, no more than ¼ inch deep. Tamp the soil lightly to prevent your seeds from washing away and give the newly seeded area a good soaking.

Seeds may be somewhat slower to germinate outdoors but should still come up in a week or two. Once your seedlings are 3-4 inches tall, you can thin them to their proper spacing, which should be at least 12 inches apart.

If you’d like to plant fennel in containers, choose ones that are on the deeper side, since plants develop a long taproot. Make sure there are drainage holes in the bottom and use a good quality potting soil.

How to Plant Fennel

When to Plant

Herb fennel should almost always be planted in the spring. You can transplant seedlings to your garden after the danger of frost has passed.

Because it needs cool weather to grow well, Florence fennel has two main planting times: early spring or late summer.

You can get your Florence fennel plants in the ground early in spring but after the danger of frost has passed. Most growers find that spring fennel crops are more likely to bolt and generally produce smaller bulbs compared to fall crops.

If you want larger bulbs and fewer pests, plant seedlings in late summer or at least 8-10 weeks before your first frost date in the fall.

Where to Plant

Both types of fennel prefer a full sun location, although they will tolerate partial shade, especially in hotter areas.

Soil should be well-drained and preferably fertile, particularly if you are growing fennel for the bulbs. Make sure you add any amendments, like compost or fertilizer, before planting.

Keep in mind that herb fennel can grow very tall (up to 5 feet), so plant it where it won’t shade anything else out.

Fennel grows the best if given its own space to occupy and placed in full sun. Afternoon shade can be helpful if you live in a hot climate, but otherwise, give plants lots of sun.

Also, certain compounds in the plants can inhibit the growth of other crops, especially tomatoes and beans. It might be a good idea to simply give fennel its own spot in your garden.

Planting Method

As you get ready to plant, plan for a spacing of at least 12 inches between each fennel plant. Some of the largest varieties may need a bit more, but it will depend on which specific cultivar you’re growing.

Dig holes for your seedlings that are as deep and slightly wider than the root ball of each plant. Loosen the roots gently with your fingers before planting each in its own hole.

Fill back in around your seedlings with soil and firm each plant in with your hands. Water your plants in well, and you’re all finished!

Fennel Plant Care

It’s important to keep fennel evenly watered as it grows, but this is especially true once bulbs start to form on Florence types. If you let plants dry out after bulb formation, the bulbs may end up being tough or small.

An application of a light mulch like straw or pine needles can be helpful to suppress weeds and keep soil moisture more even. However, don’t apply it until the weather starts to warm and dry out in the spring. Otherwise, you’ll only invite slugs into your fennel bed.

Herb fennel won’t need much fertilizing. An application of compost or a general garden fertilizer each spring will be enough to keep it happy.

Fennel care depends somewhat on which type you are growing. For example, you should cut buds off Florence fennel to keep it from flowering but may want to let herb fennel flower to collect the seeds.

Fertilizing Florence fennel is optional but it may be beneficial. If you applied a good amount of compost when planting, that could last your plants all season. However, a potash fertilizer applied every 2-3 weeks can help with bulb formation.

Deadheading your plants after they flower is a good way to prevent herb fennel from spreading too much in your garden. Obviously, if you want to harvest the seeds, wait until after the seeds are mature to snip off the flower heads.

You’ll also want to keep an eye on Florence fennel for bud formation. The bulbs will stop growing once the plants flower (also called bolting), so clip off buds before they open. You can also buy a bolt resistant cultivar to delay flowering.

Blanching Bulb Fennel

If you are growing fennel for its bulbs, you’ll need to do something called blanching once they start to form. This shouldn’t be confused with the cooking technique, which is also called blanching, that you would do before freezing certain vegetables.

Once you see bulbs start to form at the base of the stem, mound some soil up around each one until it’s covered. This helps to protect them from sunburn and keeps their color white or pale green.

You’ll probably need to do this a couple times during the season as the bulbs will keep growing out of the soil.

Fennel bulbs retain their white color through a process known as blanching. This simply means you cover them with soil to prevent sunlight from getting to them.

Pests and Problems

The great news is that fennel rarely suffers from pest or disease problems. Occasionally, you’ll get something like whiteflies or aphids on your plants, but they rarely appear in large enough numbers to do serious damage.

One critter that you may see is the swallowtail caterpillar, also known as the parsley worm. It will eat through the foliage of your plants before making a cocoon and eventually turning into a swallowtail butterfly.

If you can, allow these caterpillars to share your plants, since they are beneficial insects!

Harvesting Fennel

Harvesting herb fennel is very easy. You can simply clip off foliage throughout the season as needed, always leaving at least ½ of the plant intact. The flowers are also edible and can be added to a salad or deep-fried.

Fennel seeds will be ready to harvest in late summer or early fall. Once you see the seeds start to turn brown, clip off the entire flower head and bring it inside.

Finish the drying process by placing the flower heads on a tray or wire rack. Leave them somewhere dark and warm with good airflow. The drying process should take about 2 weeks, and you can then store the seeds in airtight glass containers.

Just shake or rub the flower heads over a cloth to get the dried seeds to come out.

At some point during the season, you may see this colorful caterpillar on your fennel leaves. Once it eats enough, it will cocoon and eventually hatch into a beautiful swallowtail butterfly, a welcome addition to any garden.

Fennel bulbs are usually ready to harvest in 70-80 days. As a general rule of thumb, harvest them when they reach the size of a tennis ball, although you can harvest small ones as baby bulbs.

Use a clean knife to slice off the bulbs at or just above ground level. Then, cut the foliage off, leaving about 3-6 inches of stem above each bulb. Trimmed like this, fennel bulbs will keep in the refrigerator for a few weeks.

Make sure that you harvest any fennel left in the ground before a frost comes through to make sure you don’t lose any of your crop.

Enjoying Fresh Fennel

Fresh fennel has a fantastic taste, and once you feel comfortable growing it, you’ll be able to enjoy it every year! You can even plant two crops of bulb fennel each year to get twice as much.

If you enjoy growing fennel, try some other easy to grow vegetables, or plant some companion herbs like dill, cilantro, or lemon thyme.