Growing Lobelia: A Complete Profile
By Growing Home Greens Team · August 19, 2026
Lobelia is the smallest flower in this dataset and one of the earliest to start indoors, which catches people out.
Eight to ten weeks indoors for a six inch plant
Botanical Interests puts the indoor start at eight to ten weeks before your last frost date. That is the same head start a tomato gets, for a plant that finishes under eight inches. The reason is germination speed rather than final size: seed takes 15 to 20 days to come up, and the seedlings are slow and tiny for weeks after that.
Seed needs light to germinate, so press it into the surface and do not cover it. Sowing in small groups rather than singly is standard practice for this plant, and the recommendation is groups of 12 to 20 seeds spaced four to six inches apart. A pinch of seed per cell is easier than trying to place something this fine.
Front row or nothing
At four to eight inches with a trailing habit, lobelia loses any contest for light. It works as an edging square, a front row, or spilling over the side of a container, and it fails anywhere it has to compete. The designer places four per square foot on that basis.
It takes full sun to part shade, which makes it one of the few options for a bed edge that only gets morning light.
Perennial somewhere, annual here
Lobelia erinus is a perennial in zones 10 and 11 and an annual everywhere else, which is how our calendar treats it. In hot summers it often stops flowering in mid-season and picks up again when nights cool. Shearing it back by about a third when it stalls is the usual response.
Source: Botanical Interests, Crystal Palace Lobelia.
Companion planting guide for Lobelia · Find your planting dates
Typically viable in USDA zones 3-10. Not sure your zone? Look it up by ZIP code.
Method: Start indoors: Start indoors from seed, then transplant outside. Lobelia needs a head start most zones can't give it by direct-sowing alone.
See exact start-indoors, direct-sow, and harvest dates for your zone →
How many Lobelia plants fit per square foot?
Spacing: 4 per square foot (square-foot-gardening spacing). Mature height: 4-8 in.
Where to buy lobelia seeds
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See it growing
Search YouTube for "growing lobelia" videos → (a live search, not a hand-picked video; results vary and aren't vetted by us)
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Growing Lobelia: A trailing edging annual under eight inches tall, started indoors far earlier than its size suggests.
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