8 Fast-Growing Gap-Filling Flowers to Plant in May – These Easy Border Stars Thrive on Neglect and Brighten Your Beds All Summer Long

In certain areas of my life, I’m a sucker for the slow burn, and love having things to look forward to – but when it comes to gardening, I’m passionate about the quick and easy wins, because I totally appreciate how growers feel time-strapped these days, and I get that the magic phrase “free time” can often feel like an optical illusion. With time being such a precious commodity, the prospect of filling empty beds and bare patches might feel overwhelming. But it doesn’t have to be hard work, believe me.

You can plug those garden gaps and create a florally fabulous landscape in weeks. The beauty with these speedy gap filling flowers is that they prefer a bit of neglect and are happy being left to run their own flower show. These plucky beauties don’t require special conditions under ground and don’t have outrageous weather needs. All these easy flowers need is what you have: bare patches and a tiny bit of time. Give them that, and they will transform awkward gaps into vibrant tapestries of color.

And just because they are good at filling gaps, that doesn’t mean they are humdrum. These zingy flowering dynamos are some of the most desirable ornamentals you can grow, offering a mix of dimensions and tones, from towering lace to ground-hugging neon fireworks. And they don’t just burst with color and disappear a few days later. They may show up fast, but they maintain an exceptional flowering marathon that stretches right through to fall. So if you’ve still got gaps to fill, rejoice! These gorgeous fast gap fillers will knit your garden together in record time, and they are raring to go right now. All you have to do is enjoy the show.

Choosing Fast Growing Gap Fillers

Tapping into your yard’s unique personality does help when mulling over these fast growing fillers. But it’s quite easy to prep your beds and borders, and you don’t need loads of skill or special tools to get the best from them. These 8 easy gap fillers aren’t overly fussy and won’t wilt if you forget the watering can. Just think about your space before you sow. Most of these high-speed bloomers are fine in USDA zones 4-9 and many can cope with some partial shade, but the more sunshine you can give them, the better. Whether you’re looking to fill a sunny border space or a vertical gap (empty spaces on a trellis or a bare fence line), there’s something here to suit.

A little ground-level prep goes a long way. Checking your soil’s moisture retention and pH levels is key, and the speed of growth you can enjoy with these plants means you have a little time to make small soil amendments if needed. Before sowing, just carry out a quick check of your soil using a meter like the XLUX Long Probe Soil Meter from Amazon and add with a little coarse sand if your soil is claggy.

Even if your tests don’t flag up problems, I’d advise applying a gentle, diluted seaweed treatment like Neptune’s Harvest Liquid Seaweed from Walmart before you fill gaps with flowers from my list. This will prime your planting spot ahead of the mammoth flowering marathon, and provides a wealth of trace minerals these flowers love. Seaweed strengthens the plant's cellular structure, making them more resilient to pests and heatwaves. This quick ritual ensures the soil is pre-loaded with goodness, so you can walk away once the roots have anchored.

Planting in May takes advantage of the warming soil, allowing nature to do most of the rest. These varieties are wired for speed, either because they are annuals with a mission to max out for one season, or because they are vigorous perennials that use runners to claim territory. In the first few weeks, a regular deep watering is the only thing required to help them settle in. Once they have established their root system, they will be reassuringly self-sufficient and ready to erupt in a riot of color in weeks.

1. Zinnias

Dynamo zinnias (Zinnia elegans) have a justified reputation for fast growth, and make gorgeous ornamentals if you’re after vibrant colors at speed. These plucky annuals are perfect for plugging awkward mid-border holes with crayon-bright tones, as well as a range of petal textures. Wherever a patch of bare dirt needs a bit of wow factor, zinnias deliver with gusto. These heat-lovers thrive in zones (2–11), leaping from seed to bloom in 60-80 days. Once they hit their stride, they create a continuous parade of color for 3-4 months, standing proud until first frost.

These curb appeal favorites only need full sun and well-drained soil to truly explode. I space my zinnias 8-10 inches (20-25cm) apart, as the natural huddle this creates helps them to support each other as they grow. Give individual plants a pinch at 12 inches (30cm) tall, as this encourages branching out, doubling your blooms. These easy gap fillers are cut-and-come-again kings, and every flower you snip triggers more. Talk about the gift that keeps on giving! You can buy gorgeous zinnias at Burpee, including ‘Queeny Lemon Peach’ and ‘Zinderella Lilac’ varieties.

2. Nasturtiums

These vigorous scramblers fill a bare patch of earth almost as quickly as your back is turned. Within 50-60 days of sowing, a dusty gap becomes a lush, lily-pad-like carpet of emerald-green leaves, soon followed by spicy-scented, jewel-toned blooms. Nasturtiums are tender perennials but are grown as annuals in zones 2–11, and provide a relentless show to first frost. Whether you go for mounding types or trailing varieties, they excel at filling a space with character and color. What’s more, they are hooked on neglect. They produce more flowers in poor, lean soil. If you feed them too much nitrogen, they’ll grow massive leaves but hide their blooms.

They crave full sun, but appreciate some afternoon shade in hotter climates. For an instant knit, sow directly into the ground, 10-12 inches (25-30cm) apart and an inch deep. They hate being moved, so you can virtually leave them to it. They don't even require deadheading to keep their 5-month flower show going. For a stunning visual mix, try the marbled foliage of 'Alaska Mix', the moody dark tones of 'Black Velvet', or the classic trailing 'Jewel of Africa'. There are some stunning nasturtiums available from Eden Brothers, including ‘Jewel Cherry Rose’ and peachy ‘Vesuvius’ varieties.

3. Cosmos

Dreamy cosmos (Cosmos bipinnatus) are the queens of elegance, and love to fill large, vertical visual holes at speed. They are capable of reaching 3-4 feet tall (90cm-1.2m). Even before the blooms arrive, they produce clouds of airy foliage that masks bare fences or leggy shrubs with a soft, green screen. They take a little longer to flower (typically 70-90 days from seed) but they are relentless once they start. You can expect a prolific display of daisy-like blossoms from mid-summer to first frost). Thriving as annuals in zones 2–11, they are perfectly happy when you do less.

Go easy on the fertilizer, or avoid it altogether. Cosmos prefer poor, sandy soils, so add a little coarse sand to the planting hole to create this texture. Tuck into your brightest spots, and for a seamless gap-filling effect, sow 12-18 inches (30-45cm) apart. Cosmos will appreciate being planted in drifts where they can lean on their neighbors for support. They are another classic cut-and-come-again plant, so the more you snip, the more the plant branches out to fill the space. For effortless charm, try the classic 'Sensation Mix', ruffled 'Double Click' or dwarf 'Sonata' varieties, all available from Burpee.

4. Bee Balm (Monarda)

If you have a sizable gap that needs a shot of high-octane energy, these perennials will give you a festival of colorful fireworks, exploding mid-to-late summer in shades of electric scarlet, violet, and pink. Even when not in bloom, the foliage is a sensory treat. I love the dark green, lance-shaped leaves which release a lovely citrus scent. And of course, this filler is so popular with hummingbirds and butterflies. Hardy in zones 3–9, bee balm spreads via underground runners. This means it doesn't just sit in a tidy clump but hunts for bare soil, knitting together to fill a large border gap within a single season. While seeds can take over 90 days to truly establish, buying nursery starts can provide mid-summer bloom that lasts for up to 8 weeks.

The biggest challenge for monarda is powdery mildew, so plant them in a spot with excellent air circulation. Space your bee balm plants 18-24 inches (45-60cm) apart, and they will fill the negative space in no time. They thrive in full sun to partial shade, and you can virtually leave them to it once established, as their mint-family genes make them tough as nails. Just deadhead spent blooms to trigger a secondary flush. For the easiest care, look for mildew-resistant varieties like the 'Sugar Buzz' series, the compact 'Pardon My Purple', or vibrant magenta Bee Balm ‘Be True’ plants from Amazon for a headstart on a spectacular season.

5. Hardy Geranium ‘Rozanne’

The hardy geranium (aka cranesbill) is a genuine speed superhero that can knit together disparate ornamental plants with seamless charm and beauty. While many perennials bloom in a single, short burst, hybrid 'Rozanne' pumps out violet-blue flowers from late spring to first frost. A single nursery start planted in May can easily sprawl to 2-3 feet (60-90cm) wide in its first season, weaving its way between leggy shrubs and taller perennials to hide bare soil with a dense canopy of mounding foliage. Hardy in zones 4-8, these plucky cranesbills are happy in full sun and partial shade and just get better and better as they knit your border together.

Because this plant is sterile, it never wastes energy making seeds, and puts all its power into a 6-month flowering frenzy. It requires no deadheading and is unfussy about soil, though it appreciates a bit of organic mulch to keep its roots cool. While 'Rozanne' is the undisputed speed champion, other quick-filling relatives like 'Johnson’s Blue' or magenta 'Max Frei' offer similar fast carpet effects. If the plant gets floppy in late August, a quick snip prompts a fresh flush in days. You can buy Geranium ‘Rozanne’ from Nature Hills.

6. Coreopsis

My next pick is a true sunshine superstar, and perfect for sun-baked gaps. Coreopsis (aka tickseed) offers a dense carpet of sunny yellow, bronze, or bicolor blooms that shimmer like golden stars. The verticillata varieties offer delicate, thread-like foliage, while grandiflora types provide a sturdier, leafy base that rapidly mounds to fill horizontal gaps. Ideally suited for zones 4-9, they are fine with a weekly watering once established. For an immediate fast fix, plant nursery starts in May. That said, annual varieties can flower in 60 days, providing a relentless display for up to 4 months.

Shearing plants back by about a third after the first flush of flowers encourages a fresh burst of foliage and a secondary wave of blooms that carries you right into early autumn. Coreopsis craves full sun and thrives in poor, rocky, or sandy soil. There are some delightful zingy yellow blooming varieties. Look for award-winning 'Early Sunrise', airy 'Moonbeam', or vibrant 'Jethro Tull' with its fluted petals. One of my favorites is the peachy bronze ‘Creme Caramel’, available from Nature Hills.

7. Bishop’s Flower

Bishop’s flower (Ammi majus) is a beautiful paradox in a border – looking like some delicate lace sculpture, but surprisingly tough and self-reliant. Often mistaken for its wild cousin, Queen Anne’s lace, this garden-friendly version is just as gorgeous. It sends up sturdy, branching stems topped with frothy, lace-like umbels that swoon in the breeze. Even before the flowers appear, the foliage masks mid-border gaps. While it can take 80-90 days to flower from seed, you can buy nursery plants or plugs to seal the deal far quicker. Its high-impact flower towers last for 10 weeks. Hardy in zones 2–11, it brings height and light with easy grace.

To speed up the clock for a May sowing of this lacy survivor, you can also soak your seeds in warm water for 12 hours before planting. Once they establish, they are very self-sufficient, and the more you snip these elegant clouds for your vases, the more the plant sends up fresh lateral shoots. Look for varieties like 'Queen of Africa' or 'Graceland', which are bred for more prolific flower heads. Or for a delicate chartreuse cloud, buy Bishop’s Flower ‘Green Mist’ Seeds from Eden Brothers.

8. Lantana

For a speedy neon color pop in sun-drenched gaps, lantana gets to work fast. These plucky bloomers can spread 3 feet (90cm) wide in a single season. Lantana plants produce tight clusters of kaleidoscopic flowers in electric shades of orange, hot pink, and sunshine yellow, often all on the same plant. Even before those intense colors start stealing the show, the sage-green foliage provides a dense, aromatic filler. Perennial in zones 9–11 and terrific fast-growing annuals in the rest of the country, a nursery start can become a full-border beauty in just 60 days.

Lantanas dislike pampering and flower better in poor soil. They are drought-tolerant once those roots are anchored, and require zero deadheading. For a quick knit in a large gap, space plants 18-24 inches (45-60cm) apart. A little heads up to pals in the deep South and Hawaii: since lantana is a vigorous grower, it is viewed as invasive in some areas. But you can get sterile, non-invasive cultivars. Try the 'Bandana' series for its mounding habit, the 'Luscious' series for vibrant bicolors, or the trailing 'Confetti' types. Just give them a hot, sunny spot and a light mulch to settle them in, then watch your garden glow for almost half a year. You can buy Luscious Royale Pina Colada Lantana from Fast Growing Trees.

Shop Stunning Gap Fillers

Turning a bare patch of earth into a vivid floral haven is easy-breezy. By selecting these plucky, fast-growing flowers, you can create a seamless and sophisticated border in weeks. These curated high-impact picks settle fast, flourish easily, and thrive with minimal fuss, ensuring your yard remains gorgeous all season long.

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