Lobelia Care: Watering, Feeding and Surviving a Hot Spell

Never let it dry out, and what happens when you do

Lobelia has no drought strategy at all. The stems are thin and soft, the leaves are small and thin, and the root system is a shallow fibrous mat that dries out from the top down within hours. Unlike plants that wilt dramatically and bounce back when watered, lobelia dries from the inside of the clump outward, and the damage is done before the outside looks bad. By the time the surface of a basket looks parched, the center of the plant is already brown and dead.

In a hanging basket in warm weather this means daily watering as a minimum, and twice a day in a hot dry wind. There is no version of lobelia in a basket that copes with being watered every other day in summer. A single hot afternoon with the compost dry will take out the middle of every plant in the basket while the trailing tips still look green, and once that center is gone the basket never looks the same again that season.

Water thoroughly, not frequently and lightly. Fill the top of the basket and let it soak in, then fill it again until water runs freely from the base. A splash that wets the top inch (2.5 cm) encourages the roots upward into the layer that dries first and makes the problem worse. If water runs straight through in a couple of seconds, the compost has shrunk away from the sides of the container and the root ball is dry inside; take the basket down and stand it in a bucket of water for twenty minutes to rewet it properly.

The midsummer slump is a heat problem, not a disease

At some point in high summer a lobelia that has been flowering beautifully will go patchy brown in the middle, stop producing flowers and look generally finished. Almost everyone reads this as disease or old age and pulls the plant out. It is neither. Lobelia is a cool-season plant that flowers hardest when days are warm and nights are cool, and it shuts down when nights stay above about 68F (20C) for a run of days, especially if that heat comes with dry air.

This is why lobelia is superb in a cool maritime summer and difficult in a hot continental one. Where nights stay hot for weeks on end, the plant will not perform through the middle of the season no matter how well you water it, and the sensible approach is to treat it as a spring and autumn plant, expecting a pause in the hottest six weeks. Where nights drop into the fifties Fahrenheit (10 to 14C), it will flower without a break from early summer until frost.

The other half of the fix is position. Full sun all day is fine in a cool climate and is punishing in a hot one. In a warm region, give lobelia morning sun and afternoon shade, or a lightly shaded position all day, and keep it off paving and walls that radiate stored heat into the evening. A basket hung on a south facing wall is the worst possible place for it in a hot summer and one of the best in a cool one.

Record the week the browning starts and the night temperatures around it. After two seasons you will know precisely which weeks your lobelia sits out, and you can plan the shear back to land just before the weather turns rather than reacting late.

Shear it back and soak it: recovery in a fortnight

The response to a browned out, stalled lobelia is decisive and it works. Cut the whole plant back hard, taking off roughly half of everything, browned center and green trailing tips alike, with scissors or shears. Do not pick through it trying to save the good bits; a clean, even shear over the top of the plant is faster and gives a better shape. Then water it heavily, soaking the root ball right through, and give a liquid feed at half strength.

The plant looks wrecked for about five days. New shoots then break from the base and from every cut stem, and within ten days to a fortnight you have a fresh, dense mound covered in buds, and the second flush usually carries through to the first frosts. This is the single most useful thing to know about growing lobelia and it is the difference between a plant that is finished in midsummer and one that performs for five months.

Keep it soaked through the recovery period. The plant is rebuilding leaf and stem from stored reserves and it is more sensitive to drying out during those two weeks than at any other time except establishment. If a basket dries out once during the recovery, the regrowth stalls and you get a thin, patchy result instead of a full one.

1. Shear off about half the plant Cut evenly across the whole mound or curtain with scissors, taking browned and green growth together. Trying to selectively trim only the dead parts takes an hour and leaves an uneven plant.
2. Soak the root ball right through Water until it runs freely from the base, or stand a basket in a bucket for twenty minutes if the compost has shrunk. Dry compost under a sheared plant produces no regrowth at all.
3. Feed at half strength A balanced liquid feed at half the label rate supplies what the regrowth needs. Full strength on a plant with reduced leaf area scorches the fine roots just when they matter most.
4. Move it out of the worst heat If the basket hangs on a hot wall, shift it somewhere with afternoon shade for the recovery fortnight. Regrowth in cooler air is faster and much denser.
5. Check water twice a day for two weeks Recovery is the second most drying-sensitive stage after planting. One dry afternoon during regrowth turns a full second flush into a thin, patchy one.

Feeding, and why less is usually better

If you mixed slow release fertilizer into the compost at planting, containers need nothing else for about three months. After that, or if you did not, give a balanced liquid feed at half strength every two weeks. Plants in ground improved with compost generally need one feed around midsummer and nothing else all season.

Resist the temptation to push a slow plant with nitrogen. Heavy feeding on lobelia produces long, soft, dark green stems with the flowers spaced far apart, which is the opposite of the dense cushion of color you are growing it for. It also makes the plant noticeably more attractive to aphids and more prone to going soft and rotting at the base in wet weather. If your plants are leafy and short of flower, stop feeding for a month and pinch them back rather than switching to a high potash product straight away.

There is no need to deadhead lobelia in the ordinary sense. The flowers are far too small to pick off individually and the plant sheds them without help. Shearing serves the purpose that deadheading serves in larger bedding plants: it removes spent growth and forming seed in one pass and triggers a new flush.