After growing butterfly gardens for decades, I must sadly report that butterflies are in serious decline. At least half are missing from my remote, pesticide-free site compared to ten years ago, but I’m not giving up. I still grow milkweeds, nettles, hops and other host plants for butterfly larvae, but the most rewarding way I support butterflies is by growing their favourite nectar-producing annual flowers.
Butterflies make use of a huge range of flowering plants, but their favourites feature blossoms with small tubular florets with droplets of nectar at the bottom. A butterfly drinks flower nectar using its slender proboscis, which resembles a retractable drinking straw, so it also helps to have secure footing from flat flower petals.
A trio of summer annuals – cosmos, Mexican sunflowers, and zinnias – provide exactly what butterflies want, and all three are cheap and easy to grow from seed.
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Cosmos florets are loaded with sweet nectar for butterflies
Colourful Cosmos
Two different species of cosmos are valued nectar plants for butterflies. Sometimes called cosmea or Mexican aster, garden cosmos (Cosmos bipinnatus) blooms in shades of pink to white. The blooms of yellow or sulphur cosmos (C. sulphureus) range from yellow to dark orange.
A study from the UK validated that garden cosmos blossoms pump out huge amounts of sweet nectar for butterflies. The revered old 'Sensation' variety produces scads of pink flowers on chest high plants, while almost red 'Rubenza' is a slightly smaller, bushier plant. Among sulphur cosmos varieties, the 'Bright Lights' blend is hard to beat.
Cosmos Growing Tips
Garden cosmos tolerates cool weather, so seeds can be started indoors a few weeks before your last frost. Slight spring chilling can promote strong early blooming. Begin direct-sowing sulphur cosmos after the soil warms. More seeds of fast-growing sulphur cosmos can be sown in early summer for autumn bloom.
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Mexican sunflower (Tithonia) is a true butterfly magnet
Robust Mexican Sunflowers
Commonly called Mexican sunflower, Tithonia rotundifolia is a phenomenal bloomer for butterflies. Growing to six feet (1.5 m), the branching plants bear bright orange blossoms for several weeks in mid to late summer. The Red Torch variety is actually dark orange, and butterflies can’t get enough of its sweet nectar. Slightly hairy Mexican sunflower leaves are moderately resistant to browsing by deer.
Mexican Sunflower Growing Tips
Start seeds indoors on the same schedule as tomatoes, and set seedlings out under cloches after the last frost has passed. Slight exposure to cool conditions encourages early blooming. To keep the big plants upright in storms, set out seedlings in groups of three, and surround them with a wire cage. When mature, they will resemble a big flowering shrub.
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The best zinnias for butterflies have exposed florets that open gradually over several days
Best Zinnias for Butterflies
Easy to grow and available in a huge range of sizes and colors, zinnias are foundation plants in any butterfly garden. As a rule, the best zinnias for butterflies have single flowers so the florets are easy to reach. Zinnia 'petals' are actually dry bracts which frame the blooming parts in the flowers’ centres. Too many bracts, as seen in double pompom zinnias, interfere with butterfly feeding.
There has not been much research on the best zinnias for butterflies, though one study showed that the small-flowered 'Lilliput' variety attracted many more butterflies compared to several other varieties. An heirloom mix dating back to the 1870s, Lilliput is a blend of small-flowered zinnias in different colours, which makes them fun cut flowers, too.
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'Zowie Yellow Flame' zinnia being visited by a tiger swallowtail
Also of interest are Zahara zinnias, which are interspecies hybrids between common zinnia (Z. elegans) and narrow-leaf zinnia (Z. angustifolia) developed at the University of Maryland in the 1980s. Butterflies seem to like the small flower size of the compact Zaharas, especially single-flowered selections like 'Zahara Starlight Rose'.
Large-flowered zinnias have produced variable results in field studies, and it is possible that some strains put so much energy into huge, petal-packed blossoms that they have little left for nectar. Not so with Zowie Yellow Flame, which features a prominent ring of florets nestled into semi-double blooms. Swallowtails and other big butterflies are constant visitors to the showy flowers.
Zinnia growing tips
Take your time getting zinnia seeds started in spring, because they need warm temperatures to make good growth. Fertile soil results in more robust plants with better quality nectar. Clipping off spent flowers helps prolong the bloom time of zinnias.
