The Fastest Way to Fill Gaps in Your Garden in May − 7 Flowers That Grow in Weeks

With summer rapidly approaching, many of us will be looking at our gardens and wondering what we could put in some of the gaps. Growing annual flowers from seed is an ideal way to fill any spaces. Providing color and interest, annuals are ideal for both large garden borders and patio containers, and when grown from seed, are very cost-effective.

As a professional gardener, I have been sowing annuals from seed for decades, both for ornamental displays and for sale as cut flowers. Generally, sown earlier in the spring undercover, May is the ideal time for direct sowing of flowering annuals outdoors, as the soil has warmed up and the risk of frost has generally passed.

Whether you prefer soft, muted pastels or vibrant, bold colors, there is a wide array of fast-growing annual flowers to choose from. Here, I share the best annual flowers to sow in May, which will bloom and enhance your garden in just a few weeks.

Cosmos

Adored for their colorful and pollen-rich flowers that are a magnet for bees and other pollinators, cosmos have long been a firm favorite of gardeners.

Not to be confused with the equally stunning perennial chocolate cosmos (Cosmos atrosanguineus), annual cosmos varieties are available in classic pastel shades as well as brighter hues. Germinating in as little as a week, cosmos are perfect for sowing in situ and often flower within 50 to 60 days.

For the middle of a border or a cut-flower patch, you may want to grow a taller variety, such as the classic white Cosmos bipinnatus ‘Purity’, which I have known to reach 5 feet in prime conditions.

Whereas, for the front of the bed, or a small container, why not try the shorter and stunningly colored Cosmos bipinnatus 'Xanthos' available from Burpee.

Unlike those started off on a windowsill or in a greenhouse, the delicate new seedlings will be especially susceptible to slug and snail attacks and will most likely need protecting.

Zinnia

One of the quickest and easiest flowering annuals to grow, zinnias are ideal for those new to gardening or growing from seed. Originating from Mexico and the Southwestern United States, it is no surprise that zinnias favor full sun and a well-drained soil.

Being half-hardy, zinnias will not tolerate any freezing temperatures, and so must only be sown outdoors when all threat of frost has passed, and the soil is warm. Typically taking 8-10 weeks to bloom from germination, if you sow zinnias in May, you could be appreciating their colorful blooms come August.

With flower colors ranging from red and yellow to a softer lime green, why not try one of the more recent bi-colored introductions, such as the Queeny Red Line zinnia available from Burpee.

Orlaya

Although often sown in the autumn to flower earlier the following year, you can also sow Orlaya grandiflora directly outdoors in May. Commonly known as the lace flower, orlaya is one of my all-time favorite umbellifer annuals to grow and was especially popular when I grew it to sell as a cut flower.

Growing on surprisingly strong stems, it produces delicate and intricate small white blooms loved by pollinators and wedding florists alike. Reaching around 2 feet tall, it remains compact and is perfect for filling a front-of-border or container gap. Typically blooming 8-10 weeks from sowing, if sown now, you can enjoy its pure, whimsical blooms later this summer.

Thriving when grown in full sun and on a well-drained soil, you can purchase white lace seeds from Eden Brothers.

Nasturtium

Perhaps one of the easiest annuals to grow, nasturtiums are truly fuss-free and, with their large seeds, are ideal for getting children into growing as well. A fast-growing annual, the month of May is perfect for sowing them, and even if planted in June, they will still flower later that summer.

With nasturtium varieties blooming in pale yellow to sumptuous red, there is a one for every color scheme. Furthermore, their leaves and petals are a great addition to a summer salad, and growing nasturtiums in your veggie patch can help protect your crops.

Remaining compact and forming a bush shape rather than trailing, the heritage Nasturtium ‘Empress of India’, available from True Leaf Market, produces crimson-scarlet flowers that contrast strongly against its green foliage.

Sow direct in full sun and keep the soil moist, but not waterlogged, to support flowering.

Cerinthe

Better known as honeywort, Cerinthe major 'Purpurascens' is a unique, fast-growing annual prized for its arching branches of blue-green foliage and deep purple bracts. A magnet for bees and other pollinators, it grows up to 3 feet tall, making it ideal for larger gaps in a border and for cutting for bouquets or vases.

Taking around 65–70 days to flower from sowing, honeywort thrives in full sun and is drought-tolerant once established. A prolific self-seeder, once grown, you will have an endless supply of new plants every spring. With a hard outer casing, you can soak the seeds for around 12 hours, which will soften the coating and help improve germination rates.

Outsidepride Cerinthe Major seeds are available to purchase from Amazon.

Ammi Majus

For whimsical elegance, Queen Anne’s Lace (Ammi majus) is certainly one to consider. A hardy annual, you can sow it in the fall or up until May for blooms later in the year. It produces soft domes of tiny white flower clusters, held aloft on slender stems, which makes it ideal for weaving into existing borders and filling any spaces.

A wonderful addition to a cut flower vase, it was regularly requested at the flower farm I previously managed. As the seeds are tiny, sow in modules like these Bonviee cell trays available from Amazon, undercover to plant out later, or sow directly into a finely raked tilth.

Tolerating both full sun and partial shade, you can purchase Queen Anne’s Lace seeds from Burpee.

Cornflower

Also known as Bachelor’s button, cornflowers (Centaurea cyanus) bloom in as little as two months from sowing and are an ideal annual flower to sow in May. Displaying true blue flowers above silver-green stems, cornflowers look equally effective when planted in drifts or clumps of odd-numbered plants.

Disliking being transplanted, cornflowers grow best when sown directly into the ground and will still bloom the same year if planted in June.

For a great flowering display, grow in full sun and on a moderate to poor soil. You can purchase the classic blue flowering cornflower seeds from Amazon, or why not try Bachelor’s button ‘Black Ball’, also from Amazon, for its sumptuous deep purple blooms.

Having filled any gaps in the garden with these fast-growing annuals, you will want to ensure that they look their best and bloom for as long as possible. Along with providing enough moisture and nutrients to support their growth, deadheading is key to preventing plants from looking untidy and encourages more flowers to form.

However, come the end of summer, you may want to allow the last flowers to go to seed, not only so you can have free plants the following year, but because some seedheads look striking when left to stand over the winter and can be an important food source for wildlife.

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