Cambridge Blue Lobelia is a brilliant, fast-growing annual that fills gardens with delicate powder blue blooms from seed in just 70 to 79 days. This cultivar of Lobelia erinus reaches a compact 4 to 6 inches tall, making it exceptionally useful for edging, containers, and tight garden spaces. Hardy in zones 4 through 9, it thrives in full sun and prefers slightly acidic to neutral soil. The tiny half-inch tubular flowers emerge reliably in humid and temperate climates, and the plant's open-pollinated heritage means you can save seed for next year's garden.
Photo © True Leaf Market
Growing Conditions
USDA Zone
4-9
Sunlight
Full Sun
Soil
Well-drained, Fertile Loam; Moist But Not Waterlogged
Preferred Temp60-75°F
Growth Stats
6 in
Height
—
Width
18 in
Spacing
70–79
Days to Harvest
PeriodPerennial
TypeForb/herb
Plant Profile
Sunlight
Full Sun
Water
—
Zones
4-9
Size
6in H x ?in W
Growth
Perennial
Care
Moderate
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Highlights
These powder blue flowers are genuinely arresting, tiny and delicate but produced in such abundance that they completely transform a planting space. The compact 4 to 6 inch bush habit means Cambridge Blue works equally well spilling from hanging baskets or defining borders without overwhelming surrounding plants. Starting from seed indoors gives you blooms in under three months, and once established, the plant thrives with minimal fussing, even in humidity where many flowers struggle.
Common Uses
Cambridge Blue Lobelia excels in containers, window boxes, and hanging baskets where its low, mounding habit and prolific flowering create dense clouds of blue. The compact size makes it perfect for edging beds and borders, and it thrives in outdoor patio settings and home gardens across temperate regions.
Growing Timeline
First Bloom
Early Summer
First Harvest
N/a
Last Harvest
N/a
Care Instructions
Start Indoors
Start Cambridge Blue Lobelia indoors 8 to 10 weeks before your average last frost date. Maintain soil temperature between 60 and 70 degrees Fahrenheit for germination. Press seeds lightly into the surface without covering them, as they need light to germinate. Keep soil moist and provide bright light once seedlings emerge.
Transplant Outdoors
Transplant outdoors 1 to 2 weeks after your average last frost date, when soil has warmed and all danger of frost has passed. Harden off seedlings by gradually exposing them to outdoor conditions over 7 to 10 days. Space plants 18 inches apart in both rows and between plants to allow for full mature spread. In mild climates, you can also sow seeds directly in late summer for winter color.
Direct Sow
Direct sow outdoors 1 to 2 weeks after your average last frost date. In mild climates, sow in late summer for winter flowering.
Growing Calendar
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Companion Planting
Ecology & Traits
Native Species
Scientific Classification
Lobelia ×speciosa Sweet var. speciosa
Family
Campanulaceae
Genus
Lobelia
Duration
Perennial
Growth Habit
Bush
Native Status
L48 (N)