Gadenin日本語Get the appBrowning: work out whether it is heat, dryness or rot
Brown patches in the middle of a lobelia are the single most reported problem, and there are three distinct causes with different answers. Get the diagnosis right first, because shearing back a plant with root rot wastes a fortnight and pulling out a heat-stressed plant throws away a perfectly good second flush.
Heat and drying browning is dry to the touch, crisp, and starts in the center of the clump where air circulation is worst and moisture goes first, with the outer trailing stems staying green longest. It appears in a hot dry spell or after a basket has been missed for a day. The plant is not diseased and the fix is a hard shear back plus a thorough soak, after which it rebuilds within a fortnight.
Rot browning is different: it is soft rather than crisp, sits at the very base of the stems near the compost, often smells sour, and the stems collapse when pinched instead of snapping. It follows a wet cool spell, a blocked drainage hole, or planting too deep. There is no shearing your way out of this one. Check that the container actually drains, stop watering until the surface dries, and if more than a third of the crown is soft, take the plant out before it spreads to its neighbors.
| Symptom | Likely cause | Action |
|---|---|---|
| Crisp brown center, dry to touch | Heat and drying out | Shear back by half, soak, feed |
| Soft brown base, stems collapse | Crown or root rot | Improve drainage; remove if widespread |
| Whole plant gray and limp, soil wet | Waterlogging | Clear drainage holes, stop watering |
| White powder on upper leaves | Powdery mildew | Improve airflow, thin out crowded growth |
| Seedlings toppling at soil level | Damping off | Remove cover, cut watering, more light |
| Sticky leaves, clustered insects | Aphids | Wash off with water; repeat weekly |
| Ragged holes, silver trails | Slugs and snails | Protect young plants; check after rain |
| Flowering stops, plant still green | Heat above 68F (20C) at night | Shear and wait for cooler nights |
Seedling losses: damping off and slugs
Damping off is the main risk while lobelia is in the tray. Seedlings this small have almost no reserves, and a fungal infection at the soil line topples them over within hours, spreading outward from a patch until half the tray is gone. It is driven by three things together: standing humidity, low light and overwatering. Remove the propagator cover or bag as soon as the green haze of germination appears, give the seedlings the brightest position you have, and water from below only when the surface starts to lighten.
Use fresh seed-starting mix rather than reused compost from last year's pots, and clean trays. Once damping off has started in a patch there is no treatment that saves those seedlings; lift out the affected area with its compost, improve the airflow immediately, and the rest of the tray usually holds.
Slugs and snails take newly planted clumps overnight, and a young lobelia is small enough that a single slug removes the whole plant rather than nibbling it. The vulnerable window is the first two or three weeks after planting out, and it is worth protecting them through that period specifically. Check the ground after rain and around the rims of containers, because slugs climb into baskets far more readily than people assume.
Aphids, mildew and rust on established plants
Aphids gather on the soft growing tips and on the undersides of young leaves, leaving the foliage sticky and sometimes distorted. On a plant with stems this fine they are easy to miss until the numbers are high. A firm spray of water over the plant knocks most of them off, repeated every few days for a fortnight, and that is usually the whole treatment needed. Heavy feeding is the underlying cause more often than bad luck; soft nitrogen-rich growth is what attracts them, so cut the feeding rate rather than reaching for a spray.
Powdery mildew shows as a white dusty film on the upper leaf surfaces, typically in late summer when nights are cool and damp but the compost is dry. It is a signal about airflow and watering, not a reason to write the plant off. Thin out congested growth or shear the plant back, water at the base rather than over the top, and stop watering in the evening so the foliage goes into the night dry.
Rust appears occasionally as small orange or brown pustules on the undersides of leaves, usually in a still, humid spot. Pick off affected leaves as soon as you see them and improve the air movement around the plant. If it takes hold across a whole container in a wet season, replace the plants rather than trying to carry them through; lobelia is quick and cheap to raise again and the infection will keep recurring in the same crowded conditions.
Keep a note of the date each problem first appeared and where the plant was standing. Rust and mildew tend to recur in the same still corner year after year, and a two year record identifies that position far faster than trying to remember which basket went wrong.
Knowing when to give up on a plant
Lobelia is worth persevering with in almost every case where the growth is green and the base is firm, because a shear and a soak turn it around in a fortnight. The judgement call comes when the base has gone soft, when the crown is bare and woody with only a few live strands left, or when the plant has been dried to collapse several times over a few weeks. In each of those cases the plant will not regain density this season.
Pull it out and replace it. A clump raised from a spring sowing takes about four months to reach planting size, which is too long for a mid-season gap, but young plants are widely available and a replacement bought in midsummer will fill the space in three weeks. Leaving a failing plant in a container also keeps a reservoir of rot or rust next to healthy neighbors.
Where the whole planting has failed at once, look at the position rather than the plants. Lobelia will not perform on a hot south facing wall in a climate with hot nights, and no amount of watering or feeding compensates. Move the container to somewhere with afternoon shade and try again, or accept that in that spot lobelia is a spring and autumn plant rather than a summer one.