Gardening Flowers Annuals How to Plant and Grow Annual Lobelia Attract butterflies to your yard with the beautiful blue and purple blooms of annual lobelia. By Viveka Neveln Viveka Neveln Viveka Neveln is the Garden Editor at BHG and a degreed horticulturist with broad gardening expertise earned over 3+ decades of practice and study. She has more than 20 years of experience writing and editing for both print and digital media. Learn about BHG's Editorial Process Updated on June 12, 2023 In This Article View All In This Article Where to Plant Planting Tips Care Pests and Problems Propagation Types Companion Plants Garden Plans FAQ Trending Videos Close this video player There are well over 300 species of annuals, perennials, and shrubs housed within the Lobelia genus—a fact that can be confusing to gardeners looking to add a specific type of plant to their space. Among them, you’ll find perennials like the cardinal flower (Lobelia cardinalis), an upright plant with 3-foot stems and vibrant red blooms. You’ll also find downy lobelia (Lobelia puberula), a fall-blooming perennial wildflower with fuzzy leaves. The most common species of the group, however, is Lobelia erinus, a compact, herbaceous annual that features bright blue to violet blooms from mid spring to early summer. The versatility of lobelia doesn’t end there. There are also several Lobelia erinus cultivars to choose from—each with slightly different growth habits and requirements. The mounding type, often called edging lobelia, is beautiful for planting along the front of beds and borders. The cascading type spills like a sapphire waterfall from window boxes or pots. You can even find compact and trailing lobelia with magenta or white flowers. There is, in short, a lobelia for almost any landscape. It should be noted, however, that all parts of the Lobelia erinus plant (including the leaves, stem, flowers, and seeds) are considered toxic to humans, so use caution when planting annual lobelia in common gathering areas. Some species of lobelia (like Lobelia cardinalis) are also considered toxic to dogs, cats, and horses. Blue Lobelia Overview Genus Name Lobelia Common Name Blue Lobelia Plant Type Annual Light Part Sun, Shade, Sun Height 4 to 18 inches Width 4 to 6 inches Flower Color Blue, Pink, White Season Features Fall Bloom, Spring Bloom, Summer Bloom Special Features Good for Containers, Low Maintenance Zones 10, 11, 2, 3, 4, 5, 6, 7, 8, 9 Propagation Seed Problem Solvers Groundcover, Slope/Erosion Control Where to Plant Annual Lobelia Lobelia prefers full to partial sun and rich, well-drained, slightly acidic soil. If you live in an area with very hot summers, consider planting your annual lobelia in a place where it will be sheltered from direct sunlight during the hottest parts of the day. It is often used in containers and garden beds, where it will spill over the sides in a cascade of color. It also makes a great plant to place along pathways or to use as a foreground plant for other annuals and perennials. How and When to Plant Annual Lobelia Annual lobelia plants are readily available at nurseries and garden centers in the spring. If you are planting starts, you can place them in the ground in the spring after the danger of frost has passed. Just loosen up the soil and dig a hole only slightly larger than the plant’s root ball. Wiggle the plant out of its nursery pot and tease apart the roots. Place the plant in the center of the hole and fill in around the roots as you stabilize the plant. Tamp the soil down as you work to remove any air pockets and then water thoroughly. If you are planting more than one annual lobelia plant, place them 6 to 18 inches apart (depending on whether you want them as independent plants or a cohesive border). If you are growing annual lobelia from seed, you will need to start the seeds indoors approximately 8 to 10 weeks before the last frost in your region. Annual Lobelia Care Tips Lobelia erinus is a pretty, easy-to-grow summer bloomer that serves as a haven for pollinators like hummingbirds and bees. While it is grown as an annual in most regions, blue lobelia is a tender perennial in zones with mild winters (Zones 10 and 11); some cultivars may self-seed. Light Annual lobelia plants are not fussy about the sun but grow best in areas that receive 6 or more hours of direct sunlight per day. In particularly hot regions, annual lobelia plants may benefit from some partial shade to protect them from the harsh late afternoon sunlight. Soil and Water Annual lobelia plants thrive best in well-drained, hummus-rich soil that is slightly acidic soil with a pH between 6.0 and 7.0. Unlike Lobelia cardinalis—which grows beautifully in boggy soil—annual lobelia is likely to rot if soil conditions are too wet. Nonetheless, lobelia plants need evenly moist soil and may require supplemental watering during hotter weather or long stretches without rain. Temperature and Humidity Annual lobelia plants prefer moderate humidity and cool weather with evening temperatures that stay between 50 and 70 degrees. In hotter climates, without protection, the peak afternoon sun may cause annual lobelia to shrivel. They are not frost tolerant and will likely die with exposure to temperatures below 40 degrees Fahrenheit. Fertilizer Lobelia erinus is considered a heavy feeder and benefits from the application of a time-release granular fertilizer immediately after planting. For the amount to use, follow product label instructions. Then, to keep your lobelia plants lush and vibrant with blooms, keep applying a liquid fertilizer or water-soluble balanced (10-10-10) fertilizer every 2 to 4 weeks throughout the growing season. Pruning Since Lobelia erinus is considered an annual plant in most regions, pruning and deadheading are not necessary. That said, if your plants grow leggy or aren’t producing many flowers, you can cut them back halfway to encourage blooming. You can also pinch the plants back at the beginning of the growth cycle to promote bushier growth. Potting and Repotting Lobelia erinus is an extremely popular addition to container gardens—especially when it is allowed to spill over the sides of the pot. Potted lobelia plants will, however, need extra care to avoid damage from drought, overwatering, and extreme weather. Place your plants in a spot where they will receive at least 6 hours of sun each day and be prepared to shelter them from extreme heat, cold, and wind. Your container-grown lobelia plants will also need consistently moist soil but will not grow well if the soil stays soggy. When planting nursery starts in a container, choose a large container with lots of drainage holes and fill it with a rich potting mix combined with organic matter (like compost). If you are planting more than one lobelia plant or combining it with other plants, place them approximately 4 to 6 inches apart. If you need to repot your lobelia plant, try to do so on an overcast day to prevent the roots from drying out. First, create a hole in the new pot slightly larger than the plant’s root ball. Then, carefully dig up your plant and place it in the new container before filling in the soil, tamping it down, and watering the plant thoroughly. 36 Container Garden Ideas to Inspire Your Own Pretty Plantings Pests and Problems Despite their sensitivity to drought, overwatering, and extreme temperatures, annual lobelia plants are largely immune from issues with pests and diseases. Fungal disease and stem rot are common ailments (typically caused by too much moisture). Spider mites can be an issue for annual lobelia in some areas, but they can be deterred with a gentle spray of cold water or the application of neem oil or insecticidal soap. Corn earworms can also be a problem and may make the plant more vulnerable to fungal diseases. If you see them, remove them manually or use insecticide. How to Propagate Annual Lobelia Lobelia erinus is easy to grow from seed and may even self-seed in areas where it can be grown perennially. Lobelia seeds are extremely small, so purchased seeds are often pelleted (coated) to make them easier to handle. If you use pelleted seeds, make sure your soil stays moist enough after planting to dissolve the pelleted coating and ensure germination. Start your seeds 8 to 10 weeks before the last frost by preparing some pots or a grow tray with high-quality seed compost. Water the soil thoroughly and sprinkle your seeds (pelleted or not) on the surface of your prepared soil, but do not cover them. The soil will need to remain moist, but not wet, so gently mist the soil or water from the bottom up to avoid disturbing the seeds. Keep the seeds in an environment (like a heat mat or heated propagator) that stays between 65- and 75 degrees Fahrenheit. Once the seeds have sprouted and are large enough to handle (in about 2 weeks), thin them into tiny clumps about 1 inch apart. Once your seedlings are approximately 3 inches long, they can be transplanted into a container or in the ground outdoors (if the danger of frost has passed). Before you plant them outside, harden them off with gradual doses of exposure to outdoor temperatures. Types of Blue Lobelia 'Blue Wings' Lobelia Justin Hancock Lobelia tenuior 'Blue Wings' bears large blue flowers on a mounding plant perfect for hanging baskets. It grows approximately 10 inches tall and 8 inches wide in Zones 9-11. 'Lucia Dark Blue' Lobelia Justin Hancock Lobelia erinus 'Lucia Dark Blue' offers gorgeous true-blue flowers on a trailing plant that's more heat tolerant than many older varieties. It trails 6 to 12 inches and can spread 12 to 24 inches in Zones 9-11. 'Magadi Blue' Lobelia Justin Hancock Lobelia erinus 'Magadi Blue' offers better heat tolerance than most lobelias. It offers soft-blue flowers and grows 8 to 12 inches tall and spreads 6 to 10 inches in hardiness Zones 8-11. 'Magadi Blue With White Eye' Lobelia Justin Hancock Lobelia erinus 'Magadi Blue with White Eye' is a sister to 'Magadi Blue' and also has great heat tolerance. It offers bi-color, soft-blue flowers marked with a white throat. It grows 6 to 10 inches tall and spreads 8 to 12 inches in Zones 8-11. 'Riviera Midnight Blue' Lobelia Peter Krumhardt Lobelia erinus 'Riviera Midnight Blue' is a more compact cultivar that bears dark blue flowers and bronze-tinged leaves on vigorous, mounding plants. It tends to grow to only 5 inches tall and spread just 8 inches in hardiness Zones 9-11. 'Riviera White' Lobelia Edward Gohlich Lobelia erinus 'Riviera White' has similar compact growth habits to its 'Midnight Blue' relative, but bears pure-white flowers on vigorous, mounding plants in Zones 9-11. 'Regatta Rose' Lobelia Peter Krumhardt Lobelia erinus 'Regatta Rose' bears brilliant carmine-rose flowers with a white center on trailing plants that stretch 6 to 8 inches and spread 10 to 12 inches in Zones 10 and 11 . 'Regatta Sapphire' Blue Lobelia Peter Krumhardt Lobelia erinus 'Regatta Sapphire' bears clear blue flowers with a white center on trailing plants. It trails 6 to 8 inches and spreads 10 to 12 inches in Zones 10 and 11. Companion Plants for Annual Lobelia Loosestrife Cynthia Haynes These vigorous growers are beautiful additions to the garden. They vary from tall, stately plants suitable for borders to others that can be planted as creeping groundcovers. Flowers, too, vary from tight spikes of 1/2 inch to 1-inch cups carried alone or in whorls. Humus-rich, moisture-retentive soil is recommended; some varieties enjoy wet soil and ample water. Several sorts may become aggressive and need to be corralled. Note: These are not the invasive purple loosestrife, which has been banned in many parts of the United States. Firewitch Cheddar Pinks Denny Schrock The quintessential cottage flower, pinks are treasured for their grass-like blue-green foliage and abundant starry flowers. This cultivar features fringed, clove-scented magenta blossoms that appear primarily from mid spring to early summer, but may sporadically pop up in late summer and early fall. 'Firewitch' cheddar pinks are hardy in Zones 3–8 and grow just 6 inches tall. Pansy Peter Krumhardt From tiny, cheerful Johnny jump-ups to the stunning 3-inch blooms of Majestic Giant pansies, the genus Viola has a spectacular array of delightful plants for the spring garden. They're must-haves to celebrate the first days of spring since they don't mind cold weather and can even take a little snow and ice! They're pretty planted in masses in the ground, but also cherished for the early color they bring to pots, window boxes, and other containers. By summer, pansies bloom less and their foliage starts to brown. It's at this time that you'll have to be tough and tear them out and replant with warm-season annuals, such as marigolds or petunias. But that's part of their charm—they are an ephemeral celebration of spring! Garden Plan for Blue Lobelia Moon Garden Plan Illustration by Gill Tomblin While it looks beautiful during the day, a moon garden is designed to be enjoyed after the sun goes down when night-bloomers such as moonflowers and four o'clocks spring to life. Lobelia erinus makes an appearance in a container alongside angel's trumpets, another night-bloomer with pendulous perfumed blooms. See This Plan Frequently Asked Questions Why is my annual lobelia not flowering? If your lobelia is slow to bloom or not flowering at all, check the moisture levels of the soil as lobelia plants prefer consistently moist soil. If you haven’t already done so, you may need to fertilize your plants every couple of weeks to encourage abundant blooming. Your plants may also take a break from blooming during especially hot weather. If you can’t offer them shelter from the harshest sun, cut the plant back by half and continue with a regimen of regular watering. As temperatures drop, it should start blooming again. Since annual lobelia can self-seed, should I worry about it becoming invasive? In perfect soil and climate conditions, lobelia plants can spread, but will not do so aggressively. If plants pop up in places you don’t want them, simply pluck them out of the ground. Was this page helpful? Thanks for your feedback! Tell us why! Other Submit Sources Better Homes & Gardens is committed to using high-quality, reputable sources—including peer-reviewed studies—to support the facts in our articles. Read about our editorial policies and standards to learn more about how we fact check our content for accuracy. Lobelia erinus. North Carolina Extension Gardener Plant Toolbox. Lobelia. ASPCA Toxic and Non-Toxic Plants.