Tips for Planting Biennial Plants

From nutrient-packed veggies like broccoli, carrots, and kale, to delicate flowers like hollyhocks and forget-me-not, biennial plants give you two years of beauty and a bounty of fresh superfoods. But before you buy biennials, here are some tips for planting biennials so they start out strong.

What is a biennial plant?

A biennial is a plant that takes two years to complete its life cycle. Biennials are usually vegetables or flowers that use the first year to establish strong roots and rosettes of foliage before putting on a spectacular show of blossoms and fruits in the second year. In the fall of the second year, biennials produce seeds and die. 

These seeds germinate the following spring for fresh biennial seedlings, and the two-year cycle continues.

Biennials are a happy medium between annuals and perennials. Annual plants display beautiful flowers but only grow for one year, while perennial plants live for years but take a long time to establish roots and begin blooming. Biennials are hardier and longer-lasting than annuals, while still providing more instant blossoms and fruits than perennials. In fact, many biennial vegetables are harvested during their first year. 

Popular biennial plants

Itching to fill your veggie garden or start a fresh flower bed? Biennials are your new lawn superheroes. Here are some of the top veggie and flower biennials for a delicious, stunning summer season.

Biennials for your vegetable garden: 

• Beets
• Broccoli
• Brussels sprouts
• Carrots
• Celery
• Collards
• Dill
• Fennel
• Kale
• Onions
• Parsley
• Rutabaga
• Swiss chard

Biennials for your flower garden: 

• Black-eyed Susans (can grow as biennials or short-lived perennials)
• Canterbury bells
• Evening primrose
• Foxglove
• Forget-me-not
• Hollyhock
• Lunaria (also known as “money plant”)
• Scorpion grass
• Stock
• Sweet William

When to plant biennials

Most biennials should be planted in early spring as soon as the ground is workable, but different biennial plants have specific windows for optimal planting. 

For example, cool-season broccoli and kale should be sown before the last frost of spring, while warm-weather biennials like sweet William and dill should be planted in late spring or early summer, after the threat of frost has passed. 

If you live in a warmer southern climate, you can sow some biennial seeds in early fall for a winter or early spring harvest. If planting in fall, sow seeds at least six to eight weeks before the first predicted frost to ensure roots are established before the cold snap. 

Check the growing instructions for your specific biennial plant before sowing seeds. Below are timing guidelines for popular biennial vegetables.

Biennial vegetable timing

Beets

• Sow seeds outdoors in spring two to three weeks before the last expected frost date. Avoid starting beets indoors, as transplanting beets and other root vegetables can be troublesome. 
• Sow seeds from midsummer through early fall (four to six weeks before your first fall frost) for a fall harvest. 
• If you live in zone 9 or warmer, you can plant beets in fall for a winter harvest.

Broccoli

• Sow seeds outdoors in spring two to three weeks before the last expected frost date, or start indoors six to eight weeks before the last frost date. 
• Sow seeds in midsummer for a fall harvest. 

Brussels sprouts

• Sow seeds outdoors in spring after the threat of frost has passed or start seeds indoors two to three weeks before the last spring frost.
• Sow seeds in midsummer for a late fall to early winter harvest.

Carrots 

• Sow seeds outdoors in spring two to three weeks before the last frost date. Do not start carrots indoors and attempt to transplant them, as it disturbs their roots.
• Sow seeds in midsummer about 10 weeks before the first fall frost for a fall harvest.

Celery 

• Sow seeds outdoors in spring eight to 12 weeks before the last frost date, or start seeds indoors about 12 weeks before the last spring frost and transplant them two weeks before the last frost date. 
• In warmer climates, plant celery in midsummer for a late fall to winter harvest. 

Collards

• Sow seeds outdoors in spring three to four weeks before the last frost date, or start indoors eight weeks before the last frost date. 
• For a late fall harvest, sow seeds in late summer, six to eight weeks before the first frost of fall.

Dill 

• Sow seeds outdoors in spring after the threat of frost has passed. Dill does not transplant easily, so do not attempt to start seeds indoors unless you plan to grow them inside for their full lifespan.
• Plant seeds every two to three weeks through midsummer for a steady summer and fall harvest.

Fennel 

• Sow seeds outdoors in spring after the threat of frost has passed, or start seeds indoors four weeks before the last spring frost date. Soak seeds one to two days before planting. 

Kale 

• Sow seeds outdoors in spring 3 to 5 weeks before the last frost date, or start indoors 4 to 6 weeks before the last frost date. 
• For a fall harvest, plant kale in late summer, 6 to 8 weeks before the first predicted frost.
• If you live in a warmer climate (USDA zone 8 or warmer), you can plant into early fall for a winter harvest. 

Onions

• Sow seeds outdoors in spring 4 to 6 weeks before the last frost date, or start indoors 8 to 10 weeks before the last frost date. 
• For larger bulbs and an earlier harvest, sow onions outdoors in fall, 6 to 8 weeks before the first predicted frost. Your onions will grow strong roots in fall, go dormant over winter and resume robust growth in early spring. 

Parsley

• Sow seeds 3 to 4 weeks before the last frost date, or start seeds indoors 6 to 8 weeks before the last frost date. Soak seeds 1 to 2 days before planting. 
• If you live in a warmer climate (USDA zone 7 or warmer), you can plant parsley in fall for a late spring harvest.

Rutabaga

• Sow seeds outdoors in early summer to midsummer (about three months before the first frost) for a fall harvest. 
• In warmer climates, you can plant rutabagas in early spring for a summer crop.

Swiss chard

• Sow seeds outdoors in early spring 2 to 3 weeks before the last frost date, or start seeds indoors about 8 weeks before the last frost date. Soak seeds in water for 24 hours before planting. 
• For a fall harvest, plant Swiss chard about 40 days before the first fall frost date. 

Prepare to plant biennials

Before you buy a bounty of biennial seeds or young plants, set your yard up for success by choosing the right biennials for your region and making a design plan for where different plants will grow in your yard or garden.  

– Order a soil test from your local cooperative extension. 

A soil test gives you information about your soil’s pH and nutrient levels and what type of soil amendments are necessary for healthy plant growth. Some biennials like broccoli prefer slightly acidic soil, while other biennials like sweet William prefer slightly alkaline soil. If you have alkaline soil but want to grow broccoli, you may need to spread an amendment like elemental sulfur to lower your soil’s pH.

Order your soil test in fall, so you have time to get the results, amend your soil, and let the amendments enrich the soil over winter for healthy spring planting.

– Choose plants that grow in your USDA hardiness zone. 

Some biennials like kale and rutabaga grow as biennials in warmer southern climates and as annuals in colder northern climates. If you want plants that will grow for the full two years, pick out biennial seeds suited for your USDA hardiness zone. 

– Think about sunlight and water needs. 

Most biennials prefer full sun, but some biennials like Canterbury bells and lunaria can tolerate partial shade or dappled sunlight. Design your garden so that sun-loving biennials grow in bright, open areas while shade-tolerant biennials grow under trees and in the darker corners of your yard alongside shade-loving perennials. 

To prevent overwatering and underwatering, plant your biennials in groups based on their water requirements. This eco-friendly, water-saving practice is known as hydrozoning.

– Spread compost in fall. 

Spread a 2- to 4-inch layer of organic matter like compost or manure in fall so it breaks down over the winter. In spring, it’ll give plant roots a rich nutrient boost as soon as their roots dig into the soil. Use a spade or trowel to work the compost into the soil to a depth of 4 to 8 inches. 

If you are planting your biennials in fall, spread compost in late summer, at least two weeks before planting your seeds or seedlings. 

– Plant on a cloudy day. 

Plants need sunlight to grow, but the hot sun can be stressful for seeds and young plants — like taking a big gulp of scalding coffee right after waking up. Comfortably introduce your plants to the garden by planting on a cool, cloudy day. 

How to plant biennials

Most biennials can be directly sown into the soil, or you can transplant seedlings that you have started indoors or purchased from your local garden center. Direct sowing is the easiest method (just scatter seeds and water!), but transplanting guarantees a higher success rate. Here’s how to sow and transplant biennials. 

How to directly sow biennial seeds

1. Prepare the seeds

Some biennial seeds germinate more quickly if they are soaked in hot water before planting, as the water alerts the seeds that they have enough moisture to grow. If soaking is recommended for your biennial seeds, soak them one or two days in advance. 

2. Spread the seeds

Follow the instructions on the seed packet to sow seeds in rows at the proper distance and depth. For example, broccoli seeds should be planted half an inch deep into the soil, 3 inches apart. 

Plant your seeds in a sunny location protected from high winds, especially if you are growing long-stemmed plants.

3. Tamp down the soil

Using the back of a rake, lightly pat down the soil to ensure seeds have strong contact with the ground.

4. Water seeds

Biennial seeds need consistently moist soil to successfully germinate. Water your biennials daily for the first two weeks, until most seeds have sprouted. Then, switch to watering once or twice a week, keeping the soil moist but not wet. 

5. Thin out seedlings

Once seedlings have grown 2 to 3 inches tall, use scissors or garden snippers to thin out overcrowded seedlings, following the spacing guidelines on your seed packet. For example, rutabaga seedlings should be thinned out to grow 6 inches apart, and broccoli seedlings should be thinned out to grow 12 to 20 inches apart. 

6. Spread mulch

Once seedlings are 3 inches tall, spread a 2- to 3-inch layer of organic mulch like straw or grass clippings around your biennials to prevent weeds, protect roots, and retain soil moisture, so you have healthier plants and less watering on your to-do list. 

Spread rings of mulch around your plants, making sure the mulch does not touch the stems of your biennials, as this can lead to disease, stem damage, and rot. 

How to transplant biennial plants

Transplanting requires more planning and digging than simply spreading seeds, but you won’t have to worry about seeds failing to germinate. You can grow your own seedlings or buy them from a local garden center. 

Harden off your seedlings two weeks prior to planting, and water deeply right before you begin the planting process. You don’t want your plants to be thirsty when they enter their new home!

1. Dig the first hole

It’s tempting to dig all your holes at once, but only dig holes one or two at a time to prevent the soil from drying out before you plant each biennial. Use a shovel or spade to dig the hole twice the size of the root ball, slightly deeper than the plant’s pot depth.

2. Plant your seedling

Gently remove your seedling from its container and set it in the hole. Then, backfill the hole with soil. Once you’ve backfilled half of the hole, water it to settle the soil. Fill the rest of the hole with soil and gently firm the earth around the stem with your hands. 

Pro Tip: Mix compost or manure into the soil before backfilling to give your plant roots a nutrient boost as they get established. 

3. Water thoroughly

Deeply water your biennial seedlings to eliminate air pockets in the soil and acclimate your plants to their new environment. Having the right amount of water will prevent stress on the plant.

4. Mulch around your plants

Spread a 2- to 3-inch layer of organic mulch around young plants. Remember, keep the mulch away from the stems of your plants.

How to care for biennials

Once your biennials are safely in the ground, they won’t ask too much from you. However, it’s important to give them some routine TLC to encourage vigorous growth and bountiful harvests and prevent disease and stress. 

Water weekly

Water biennials daily for the first two weeks after planting. Then, transition to once a week in the cool spring weather and up to three times per week in the summer heat, keeping the soil moist (but not wet, as soggy soil can cause root rot). Biennials typically need 1 to 1.5 inches of water per week. 

Fertilize

If your biennials require nutrient-rich soil, you may need to fertilize using synthetic or organic fertilizer. Fertilizer should be applied either at planting or three to six weeks after planting, based on the plant type.

Fertilize based on your plants’ specific nitrogen, phosphorus, and potassium (N-P-K) requirements. Leafy greens like collards need high-nitrogen fertilizer (15-0-0) for plentiful leaves, while root vegetables like carrots require low-nitrogen, high-potassium fertilizer (4-4-8) so their energy isn’t diverted toward excess leaf production. 

Light-feeding biennials like kale, lunaria, and Swiss chard only need one or two applications of fertilizer throughout the growing season, while heavier feeders like onions and collards need to be fertilized more often throughout the season (every two to three weeks for onions and every four to eight weeks for celery). 

Mulch

Spread a 2- to 3-inch layer of mulch around your biennials every year, generally in mid- to late spring, to prevent weeds and disease, protect plant roots, and to help the soil retain moisture. You may need to rake up old mulch to make room for the fresh supply. 

Repel pests naturally

Instead of spraying harsh chemicals in your garden, repel insects like aphids, flea beetles, and whiteflies with organic pesticide like neem oil and Bacillus thuringiensis (Bt) spray. Install a fence or net to deter deer and rabbits. 

Also consider pest-resistant companion plants like chives, which deter Japanese beetles, carrot flies, aphids, and rabbits; lemongrass, which repels mosquitoes; and petunias, which use their sticky stamens to trap aphids, tomato hornworms, and asparagus beetles. 

Weed

Hand weed around your biennials once a week to prevent pesky plantain, dandelion, and other weed invasions. Weed in the morning when the soil is moist, so you can easily pull up weeds by the roots. 

Stake tall plants

Don’t wait for your plants to droop. Stake long-stemmed, top-heavy plants so they stand tall through the growing season. Use stakes that are about 6 inches shorter than the plant’s mature height. 

Prune and trim

Use pruning shears or scissors to cut diseased stems and leaves to protect the rest of the plant. Trim back scraggly, overgrown stems to prevent overcrowding and promote dense, even growth.

Deadhead flowers

As flowers fade, cut them back with scissors or pruning shears, or simply pinch them off with your thumb and forefinger. Deadheading encourages plants to bloom longer and produce more flowers, as they aren’t expending energy going to seed. 

If you want fresh seedlings next spring, hold off on deadheading in fall so your plants can release seed pods.

FAQ about planting biennial plants

Get the best out of your biennials

Biennials provide delicious superfoods and fabulous flowers, and designing and planting your own garden can be rewarding for the whole family. But once you’ve done all that digging, you may want a well-deserved breather.

Call a local lawn care pro to lend a green thumb for all your mulching, weeding, and trimming needs, so you can enjoy your bountiful biennial harvest throughout the summer and fall. 

Main Photo Credit: cocoparisienne | Pixabay