Planting Lobelia: Spacing for Edging, Baskets and Containers

Wait for warm soil, not just the end of frost

Lobelia is hardened enough to shrug off a light frost, but it does not grow in cold ground. Planted into soil still sitting at 50F (10C), a clump will sit motionless for three weeks, go slightly purple in the leaf, and only start away once the ground warms, by which time an identical plant put out a fortnight later has caught up and passed it. There is nothing to gain by planting early and a fair amount to lose.

The signal to plant is soil that has reached about 55 to 60F (13 to 16C) and nights that stay above 45F (7C). In practice that is a week or two after your last frost date, at the point where other bedding is going in and the grass has started growing properly. Choose a dull or showery day if you can; planting into strong sun means every clump wilts by evening and needs nursing for days.

Take the whole tray out to the site and lay the plants out on the surface before you dig anything. Lobelia spacing is much tighter than people expect, and laying out first is the only reliable way to find out you have half as many plants as you thought before you have committed half of them to the wrong end of the bed.

Spacing: much closer than other bedding

Because you planted clumps rather than single seedlings, each unit is already multi-stemmed, and the aim is for neighboring plants to knit into a continuous band with no gaps showing. For an edging along a path or the front of a border, set clumps four to six inches (10 to 15 cm) apart. That looks absurdly tight when you do it and looks exactly right six weeks later.

In containers the spacing tightens further because you are filling a defined shape in one season rather than covering ground. Three to four inches (8 to 10 cm) between plants around the rim of a basket or trough is normal, and commercial baskets are planted tighter still. The limit is not root competition so much as water: the more plants in a fixed volume of compost, the faster it dries, and lobelia has no tolerance for drying out.

Do not mix the trailing and the compact bush types in the same run of edging. The trailing sorts flop forward over the path and the bush sorts stay as neat mounds, and alternating them gives a wavy, unresolved line. Use the compact ones for edging and the trailing ones where they can fall over an edge.

Soil and compost: moisture retention is the whole job

Lobelia has a fine, shallow root system that sits in the top three inches (8 cm) of soil, so it lives or dies on how well that top layer holds water. In the ground, fork a couple of inches (5 cm) of garden compost or well rotted manure into the planting strip beforehand. On a thin, sandy or gravelly soil that step is not optional; without it you will be watering an edging every day from midsummer.

In containers, use a good all-purpose potting mix and mix in water-retaining material rather than relying on the compost alone. A third by volume of coir or fine bark, or a handful of water-storing granules per basket, makes a real difference to how long the root ball stays damp on a windy day. Do not use a very free-draining mix intended for succulents or alpines, which is the opposite of what this plant wants.

Feed is best mixed in at planting rather than applied later. A slow release fertilizer at the rate on the pack, stirred through the compost as you fill, carries container plants for roughly three months and removes the risk of forgetting to feed during the busiest part of the season. In the ground, plants in soil improved with compost usually need nothing until midsummer.

Note the compost mix and whether you added retaining granules for each container, alongside how often it needed water in the hottest week. Two seasons of that record tell you exactly which of your baskets are viable and which will always be a daily chore.

Planting the clumps, and the first fortnight

Water the tray an hour before planting so every clump comes out with its compost intact. Make a hole slightly larger than the root ball with a trowel or your fingers, set the clump in at the depth it was growing, and firm gently around the sides. Planting too deep buries the base of the stems and invites rot at the crown, which is the commonest way a healthy young lobelia is lost in the first month.

In baskets, plant the lobelia last, after the upright center plants are in, and set them right at the rim with the clump tilted slightly outward so the stems fall over the edge from the start. If you plant them flat and level with the surface, they spend three weeks growing sideways before they begin to trail, and you lose the early effect.

Water in thoroughly immediately, enough that water runs from the base of a container or soaks well down in a bed. Then keep the root zone consistently damp for the first two weeks without exception. This is the period where the plant is bridging from its cell root ball into the surrounding compost, and a single drying out now sets it back a fortnight or kills it outright. After that the roots are out and it becomes far more forgiving, though never actually drought tolerant.

1. Water the tray an hour beforehand Clumps lifted from dry cells break up and lose roots. An hour's soak means the compost holds together as you plant, and the plant is not starting from a moisture deficit.
2. Lay everything out before digging Place every clump on the surface at final spacing first. Lobelia goes in tighter than looks sensible, and this is the only way to check your plant count against the run before you commit.
3. Plant at the same depth, no deeper Set the top of the root ball level with the surface and firm around it. Burying the stem bases half an inch (12 mm) too deep causes crown rot in wet spells and is a common early loss.
4. Tilt basket plants outward At the rim of a basket or trough, angle each clump so its stems point out over the edge. It starts trailing immediately instead of spending three weeks reorienting itself.
5. Keep it damp for two weeks Check daily and water before the surface dries. This establishment period is the least forgiving stage in the plant's life; after a fortnight the root system is out into the compost and the plant is far tougher.