How to Sow Lobelia Seed: Surface Sowing Dust and Transplanting seedlings in Clumps

The seed is dust, and that decides the whole method

Open a packet of lobelia seed over a white saucer and you will see why every instruction about it is different from other bedding plants. The seed is roughly the size of ground pepper, so fine that a thousand of them barely cover a fingernail and a breath across the packet loses half of them. There is almost no food reserve inside a grain that small. A seedling has enough stored energy to push a root out and open two seed leaves, and that is all, which means it has to reach light within a day or two of germinating or it starves in the dark.

That single fact explains the two rules that matter. First, the seed goes on the surface and is never covered, not even with a dusting of compost or vermiculite, because a sixteenth of an inch (2 mm) of soil is a foot of rubble to something that size. Second, you do not thin lobelia and you do not prick it out singly, because a single seedling at that scale is a green thread you cannot hold without crushing. Sow more thickly than instinct suggests, too: a mat of seedlings is exactly what you want, because you will lift it in clumps later.

Sowing: timing, surface and moisture

Sow in late winter or very early spring, about ten to twelve weeks before your last frost date. Lobelia is slow off the mark compared with most bedding, taking roughly four months from sowing to a plant worth putting in a basket, so sowing at the same time as your fast annuals leaves you with something undersized when planting weather arrives. Indoors on a windowsill or under lights is the right place; a cold greenhouse works if you can hold it above 50F (10C) at night.

Fill a small tray with seed-starting mix, firm it lightly and level it, then water it thoroughly from below by standing the tray in an inch (2.5 cm) of water until the surface goes dark. Do this before sowing, never after. Watering from above once the seed is on top washes the whole sowing into one corner or straight down into the compost where it will never come up, and this is by far the most common way a packet is lost.

To spread the seed evenly, tip it into the palm of one hand, take small pinches between finger and thumb and rub them out over the surface from about six inches (15 cm) up, working across the tray in both directions. Mixing the seed with a teaspoon of dry silver sand first makes it visible and much easier to distribute. Do not press the seed in and do not cover it. Slide the tray into a clear bag or cover it with a sheet of glass to hold humidity, and keep it out of direct sun.

Write down the date you sowed and the date the first green haze appeared. Lobelia germination is slow enough that people throw trays out at day twelve, and having last year's number in front of you stops that happening again.

The first six weeks are slow, and that is normal

Germination takes two to three weeks at 65 to 70F (18 to 21C), and the first sign is a faint green fuzz across the surface rather than recognizable seedlings. Remove the cover as soon as you see it, because standing humidity over emerged seedlings this small brings on damping off within days. Move the tray to the brightest place you have and drop the temperature a little, to around 60F (15C), which keeps the seedlings squat instead of drawing them up into threads.

For the next month almost nothing appears to happen. The seedlings sit at half an inch (12 mm) tall with two round seed leaves and a couple of true leaves, and they look far too small to ever become a plant. This stalled-looking phase is normal and is simply the plant building a root system before it spends anything on top growth. Keep the compost evenly moist by watering from below, never let the surface dry to pale, and turn the tray every couple of days so the seedlings do not all lean one way.

Do not feed at this stage and do not be tempted to warm them up to speed things along. Heat plus low light gives you a tray of pale, leggy threads that collapse the moment you touch them. Cool, bright and steadily moist is the combination that produces the stocky little clumps you want.

Transplanting seedlings: lift small clumps, never single seedlings

When the seedlings are around three quarters of an inch (2 cm) tall and have two or three true leaves, they are ready to move. You are not going to separate them. Take a plant label or an old dinner fork and slide it under the mat, then lift a lump of compost carrying roughly five to eight seedlings growing together, holding it by the compost rather than by any stem. That clump is one unit from here on, and it will grow into one bushy plant with far more stems than a single seedling ever produces.

This is standard practice for lobelia and not a shortcut. A clump of six seedlings knits together at the base within a fortnight and behaves as a single multi-stemmed plant, which is precisely the dense cushion or curtain you are after. A lone seedling grown on by itself produces a thin, sparse thing that never fills a basket edge. Aim for clumps of similar size so your trays finish evenly.

Plant each clump into a cell tray or into small pots, setting it at the same depth it was growing and firming only enough to hold it upright. Water from below again and keep them somewhere bright and cool, around 55 to 60F (13 to 16C). Growth accelerates noticeably from this point: within three weeks you will have a recognizable little plant, and within six a mound ready for hardening off.

1. Wait for two or three true leaves Move the seedlings when they are about three quarters of an inch (2 cm) tall. Earlier than that the compost mat falls apart in your hand; much later and the roots are tangled tightly enough that lifting damages them.
2. Lift clumps of five to eight Slide a label or fork under the mat and lift a lump of compost with several seedlings in it. Hold the compost, never the stems. Do not try to tease individuals apart, and do not worry that they look crowded.
3. Set one clump per cell Drop each clump into a cell or a small pot at the depth it was growing, and firm gently around it. One clump equals one finished plant, so count your cells against how many plants you actually need before you start.
4. Water from below and keep it cool Stand the tray in shallow water until the surface darkens, then drain it. Grow on at 55 to 60F (13 to 16C) in the brightest position you have. Warm and dim at this stage produces soft growth that never recovers.
5. Start a weak feed after three weeks Once the clumps are growing away visibly, give a balanced liquid feed at quarter strength every two weeks. Full strength feed on plants this size scorches roots and produces soft growth that will not harden off well.

Hardening off before anything goes outside

Lobelia raised indoors is soft, and moving it straight outdoors bleaches the foliage white and checks growth for a fortnight. Start hardening off about two weeks before your planting date, once nights are reliably above 45F (7C). Put the trays outside in a sheltered, lightly shaded spot for two hours on the first day, then four, then most of the day, and leave them out overnight only in the last few days.

Watch the watering closely during this period. Small cells outdoors in wind dry out several times faster than the same cells on a windowsill, and a lobelia clump that dries to the point of collapse at this stage often does not fully recover. Check them morning and evening rather than once a day. The plant is ready to go out when the foliage has taken on a slightly darker, tougher look and the clump has filled its cell with root.