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Impatiens are tender perennials grown as summer annuals, and the calendar follows from two hard limits. They will not tolerate any frost at all, and they will not grow properly below about 55F (13C). That gives a season that starts later than most people expect and ends abruptly rather than fading. Between those two points they flower continuously, which is why they are worth the fuss at either end.
The season begins indoors. Sow ten to twelve weeks before your last expected frost, or take the young plants you rooted from cuttings last autumn and start growing them on. Plant out only once nights are reliably above 55F (13C), which is usually two to three weeks after the last frost date rather than on it. Impatiens put out into cold soil sit and sulk for a month and never catch up with plants put out a fortnight later into warm ground.
From then it is water, light feeding and one or two cut backs, through to a late summer flush as the nights cool, which is often the best flowering of the year. The last job of the season is taking cuttings, and it has to happen while the weather is still warm rather than in the cold fortnight before the frost.
| Stage | Timing | Main jobs |
|---|---|---|
| Sowing indoors | Ten to twelve weeks before the last frost | Surface sow on damp compost, do not cover, 70F to 75F (21C to 24C) |
| Germination | Fourteen to twenty one days after sowing | Keep bright and evenly damp; water from below |
| Transplanting seedlings | At the first true leaves | Move to plugs, handle by a leaf, grow on cool and bright |
| Pinching | At three inches (8 cm) tall | Take out the tip to force branching |
| Hardening off | Two weeks before planting out | Increase outdoor time daily; keep out of direct sun |
| Planting out | Nights reliably above 55F (13C) | Space eight to twelve inches (20 to 30 cm); water in; slug protection |
| Early summer | Plants filling out | Half strength feed every two weeks; pinch once more |
| Midsummer | Peak heat | Daily watering in containers; check leaf undersides for mildew |
| Late summer | Flush as nights cool | Cut back leggy plants by a third; take cuttings |
| First frost | Overnight | Plants blacken and die; clear and bin the debris |
Late winter to spring: starting them off
Sow indoors ten to twelve weeks before your last expected frost. The seed needs light, so press it onto the surface of damp fresh compost and leave it uncovered, hold it at 70F to 75F (21C to 24C), and be patient, because fourteen to twenty one days is normal and it can be longer. Water from below by standing the tray in shallow water, since watering over the top both dislodges surface sown seed and encourages damping off.
Transplant at the first true leaves into plugs or small pots, handling each seedling by a leaf. Grow them on cooler than the germination temperature but as bright as you can manage, because warm and dim gives long floppy seedlings that never make a decent plant. Pinch the tip out once at around three inches (8 cm) tall. Feed at quarter strength weekly from a fortnight after transplanting seedlings.
If you overwintered cuttings instead, this is when they wake up. Increase the watering as the light improves, step them up into four or five inch (10 to 12 cm) pots, feed at half strength every two weeks, and take a further round of cuttings from the shoots they produce. Those late winter cuttings root in a week and give you extra plants for the same season, which is the main advantage of the cutting route over seed.
Late spring: hardening off and planting out
Do not rush this. The temptation is to plant out on the last frost date, and impatiens punish it. Wait until night temperatures are reliably above 55F (13C) and the soil has warmed, which is usually two to three weeks later. A plant that goes out into cold soil stops growing, drops its lower leaves, and is still smaller in midsummer than one planted a fortnight after it.
Harden off over about two weeks. Start with an hour outside in a sheltered shaded spot and build up daily, bringing plants in at night for the first week. Impatiens harden off to wind and cool air, not to sun, so keep them shaded throughout; a plant moved abruptly into direct light scorches within a day.
Plant into soil with plenty of well rotted compost forked through it, spacing eight to twelve inches (20 to 30 cm) apart. The wider spacing looks sparse on the day and it is the right distance, because the gaps close within a month and airflow between plants is a genuine defense against downy mildew and gray mold. Set the top of the root ball flush with the surface, water in thoroughly once, put slug protection down the same day, and keep any mulch two inches (5 cm) clear of the soft stems.
Record the date you planted out and the night temperature at the time, alongside how the plants looked a month later. In your own garden the difference between planting on the last frost date and planting three weeks after it is dramatic, and one season of notes settles the argument permanently.
Summer: the flowering months
From early summer onward the routine is simple and largely about water. Containers and hanging baskets need daily attention in warm weather and sometimes twice a day at the peak. Plants in the ground in shade want a good soak twice a week. Feed at half strength every two weeks in containers and rather less in the ground, and resist the urge to push them, because heavy nitrogen is what stops the flowering.
Pinch or cut back once around midsummer if plants are stretching, taking a third off above a leaf pair. New shoots break in five to seven days and the plant is flowering again within two to three weeks, denser than before. This is also when to check leaf undersides weekly for the even white bloom of downy mildew, particularly in a damp spell, because catching it on the first plant is the only chance you get to remove it before it moves through the bed.
Expect a slowdown in the hottest weeks and a noticeable improvement as the nights start to cool in late summer. That late flush is usually the best flowering of the year, and it runs right up to the frost, so there is no point clearing plants early because they looked tired at the height of summer.
Late summer and the end: cuttings before the frost
The most important date in the impatiens year is not the last frost in spring but the first frost in autumn, because the plants go from full flower to black mush in a single night with no warning and no partial damage. There is nothing to salvage the following morning. Anything you want to keep has to leave the garden before that night.
Take cuttings in late summer, at least two weeks before your first expected frost, while the weather is still warm. Four to six inch (10 to 15 cm) non flowering shoot tips, lower leaves stripped, stood in a glass of water on a bright windowsill, root in five to ten days at close to a hundred per cent. Cuttings taken later, in cold damp conditions, mostly rot, which is why this job belongs in warm weather rather than in the tidy up.
After the frost, clear the dead plants promptly rather than leaving the blackened stems through winter. If any of them had downy mildew, bag that material and put it in household waste rather than composting it, rake off the fallen leaves around them, and make a note not to plant common bedding impatiens in that ground again. Tip out container compost instead of reusing it. Then the year restarts on the windowsill with the cuttings you took.
Note your first and last frost dates each year alongside the date you took cuttings and the date you planted out. Those four dates define the entire impatiens season in your garden, and after two seasons of records you will know exactly which week to do each job rather than guessing from a general calendar.