Biennial Flowers to Sow in June – 7 Low-Maintenance Plants To Fill a Crucial Gap in Your Garden
Why not get ahead and guarantee blooms for next year’s beds and containers
Biennials are the often-overlooked middle sibling of flowers. They don’t make the yearly displays of perennials or offer the immediate impact of annuals. But don't overlook how important biennials are; you should embrace them, and June is an ideal time to sow lots of biennial flowers.
Sowing biennials in early summer fits their life cycle perfectly. They’ve got many months ahead to produce strong roots and foliage, before going dormant for winter and then bursting into life and flowering in spring or summer. With a bit of planning, you can get lots of gorgeous blooms to enhance your flower beds or container displays at a crucial time, with many flowering after the spring bulbs and before the annuals.
In mild and northern hardiness zones, biennials are ideal for planting in June. You can sow seeds either indoors or outdoors, depending on your preference, and there are some fantastic plants to choose from. I prefer the former – sowing indoors and planting them outdoors in fall – and here are some of my favorite biennials to sow in June.
Foxgloves
Foxgloves, or Digitalis, bring vertical drama to any flower bed in spring and early summer, with their tall spikes of flowers demanding attention before many other perennials or annuals really get going. The flowers are good for pollinators and also look stunning in a vase when used as cut flowers.
I usually start foxglove varieties indoors in June. The seeds are very fine and only need to be scattered on the surface and pressed into the compost. Don’t cover them though, as they need light to germinate.
Give them temperatures of 50–60°F in an unheated greenhouse or cold frame, and they’ll germinate in a few weeks. Then, you can transplant seedlings into the garden in early fall.
See foxglove seeds at True Leaf Market
Canterbury Bells
Canterbury Bells is a biennial type of campanula and a popular cottage garden plant. They have fragrant, bell-shaped flowers in shades of white, pink, blue, and purple that sit on tall spires and gently nod in the summer breeze.
This biennial flower prefers milder climates, rather than the hottest, southern US hardiness zones.
To sow seeds indoors in early summer, finely scatter the seeds in trays and press them into the compost. You can top with a very fine layer of vermiculite, but the seeds do require light to germinate. At temperatures of 70-75°F, the seeds will germinate in two to three weeks.
Gently pot up the seedlings into individual pots filled with well-draining potting soil once they are large enough to handle. Plant them outside in the fall or next spring into a sunny spot in the garden.
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Wallflowers
Biennial wallflowers are popular spring flowering plants to use in large numbers for bright, highly fragrant displays. They come in vivid shades, which make a real impact when planted in mass, and are really simple to grow from seeds sown in early summer.
Wallflowers are one biennial that buck the trend for me. As I said at the start, I usually start my biennials off in trays or pots, but not when I grow wallflowers. For them, I still use the traditional way of starting them off a nursery seedbed.
To do this, I prepare the bed and rake it to a fine tilth. I then sow seeds into rows, let them germinate, and them transplant them from this bed to their final position in the fall.
You can, however, still start them off in trays if you want to and prick them out into pots to grow on before planting out. Wallflowers reliably germinate quickly and benefit from pinching out once they reach six inches tall, to encourage bushing and more blooms.
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Sweet Williams
Sweet William, or Dianthus barbatus, is a nostalgic plant, and one that I grew in large numbers for displays in one historic garden I worked in for several years. The plant played an important role in the summer formal parterre display each season, and I always sowed the seeds of the biennial in summer.
Sweet Williams burst into flower in late spring and summer. The colorful blooms, which can be red, pink, purple, or white, are clusters of many flowers, each with serrated petals, that sit atop thin stems, and attract bees and butterflies during the flowering season.
To grow Sweet Williams, sow seeds onto the surface of pots filled with compost. These pots can go into a cold frame or an unheated greenhouse to germinate. The location is important, as for the best results, Sweet Williams need cool temperatures rather than too much warmth.
If you don’t have a suitable location to start the plants off, the seeds can be sown directly into their growing position onto the soil surface and thinned to 12 inches apart.
See the range of Sweet William seeds at Botanical Interests
See the range of Sweet William seeds at True Leaf Market
Honesty
Honesty, Lunaria annua, has been popular for centuries, primarily for its translucent seedheads that can either be cut and used for dried flower arrangements or left on the plant to add glorious interest in a winter garden.
But don’t overlook the flowers, too, as the delicate four-petalled purple blooms (you can get white varieties, too) are a beautiful addition to any space in late spring or early summer.
June is the last month to sow honesty seeds each year, to ensure they have the time to develop strong roots this year to see them through winter and burst into life come spring.
Sow the seeds into pots and cover with a light layer of soil or vermiculite and germinate them in a cold frame, or make shallow drills into your beds and sow directly into them.
Whether you transplant young plants or thin seedlings, honesty plants ultimately want to be spaced 12 inches apart and grown in full sun or partial shade.
See honesty seeds at True Leaf Market
Hollyhock
Hollyhocks are grown as biennials, but once you plant them in a garden, you may consider them a perennial as they will make themselves properly at home and self-seed around. Like a guest who simply won’t leave, but one you are happy to have stay.
The plants are adored for the tall flowering spikes that can reach up to six feet in summer. They won’t flower this year, but planting hollyhock seeds in June will give you stunning blooms next year.
And if you sow next summer, they’ll bloom the following year. From that point on, your self-seeded hollyhocks can take over for year after year of charming flowers.
Sow the seeds directly into the soil this month. Gently scatter them and barely cover them with soil, as they need light to germinate. If you keep them moist, but not waterlogged, they’ll germinate readily in early summer temperatures.
Hollyhocks do make good plants for pollinators. If you do want to grow them in a wildlife garden, pick single-flowered varieties, whose nectar is easier to access by bees and butterflies.
See the range of hollyhock seeds at Botanical Interests
See the range of hollyhock seeds at True Leaf Market
See the range of hollyhock seeds at Burpee
Evening Primrose
This native plant has decorative, fragrant, bowl-shaped, yellow flowers that open in the evening and attract night-time pollinators, including moths and nocturnal bees.
It is a very pretty night-scented plant, but it has a reputation for being aggressive. Evening primrose does self-seed, so deadheading is advisable if you want to stop it from spreading around your garden or into your neighbor's property.
And you can plant it in containers to stop the rhizome spreading. Control is something to consider before sowing this biennial plant.
Directly sow the seeds outdoors onto the soil surface and tamp them down, then thin seedlings to around 12 inches apart once they germinate. If you start the plants off indoors, which you can do, sow into biodegradable pots this month and plant them out in the fall to reduce any root disturbance.
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What to Shop
This potting mix is made from natural ingredients and specially formulated for starting seeds.
Vermiculite can be used on the top of seed trays to retain moisture and warmth. It is a lightweight topping for seeds to push through after germinating.
This seed tray has 16 extra-large flexible cells that are made from silicone. The material ensures seedlings easily 'pop' out when transplanting to reduce damage or stress.
June is a busy month for sowing and planting. As well as biennials to sow in June, there are still annuals and vegetables to plant this month. For inspiration, our in-depth guide to what to plant in June contains fast annuals, heat-loving vegetables, summer bedding, herbs, and more that you can add to your garden in early summer.
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