Impatiens Seedlings and Young Plants: Getting Them Through the First Weeks

Germination is slow, and that is normal

Impatiens seed takes fourteen to twenty one days to come up and can take longer, which is slow enough that most of the trays thrown away as failures were simply not given enough time. Hold the tray at 70F to 75F (21C to 24C) and leave it alone. Germination falls off sharply below 65F (18C), so a cool windowsill or an unheated room is the usual reason for a tray that never emerges rather than bad seed.

The seed needs light to germinate, which catches people out. Press it onto the surface of damp compost with a fingertip so it makes contact, and do not cover it with compost at all. Covering it is the second most common cause of failure after cold. A clear lid or a sheet of glass over the tray holds humidity without excluding light and can come off as soon as the first seedlings appear.

Water from below throughout. Stand the tray in an inch (2.5 cm) of water until the surface darkens, then lift it out and let it drain. Watering over the top washes surface sown seed into the corners of the tray and wets the seedling stems, which is what starts damping off. Sow ten to twelve weeks before your last expected frost, which allows for the slow germination and still gives a decent sized plant to put out.

Damping off is the main way seedlings are lost

Damping off takes seedlings overnight. A tray that looked fine yesterday has seedlings lying flat this morning, each one pinched and darkened at the point where the stem meets the compost, and it spreads outward in a widening circle over the following days. It is caused by fungi in the compost that thrive in overwatered, warm, stagnant conditions, and impatiens seedlings are thin stemmed and slow, which gives it plenty of opportunity.

There is no treatment once it starts. Everything you do about it is prevention, and it is all straightforward. Use fresh sterile seed-starting mix rather than garden soil or last year's opened bag. Use clean trays, washed if reused. Sow thinly so the seedlings are not touching, because a dense mat holds moisture at the stem bases and lets the fungus move from one seedling to the next. Water from below and let the surface dry slightly between waterings.

Airflow is the other half. Take the propagator lid or cover off as soon as the first seedlings appear rather than leaving it on for the look of it, and if the air in the room is still, a small fan on low across the tray for a few hours a day makes a real difference. If damping off does start, remove the affected seedlings and the compost immediately around them at once, stop watering until the surface has dried, and increase the ventilation. You will usually save the rest of the tray.

1. Use fresh compost and clean trays Sterile seed-starting mix from a new bag, in trays washed since last use. Garden soil and old opened compost both carry the fungi that cause damping off.
2. Sow thinly Space the seed so the emerging seedlings do not touch. A dense mat traps moisture at the stem bases and lets the problem spread from plant to plant.
3. Water from below and let the surface dry Stand the tray in shallow water until the surface darkens, then drain. A permanently wet surface is the condition damping off needs.
4. Take the cover off as soon as seedlings appear Humidity helps germination and harms seedlings. Once they are up, ventilation matters more than moisture.
5. Remove casualties immediately Lift out any collapsed seedling along with the compost around it as soon as you see it, rather than at the end of the week.

Transplanting seedlings and growing on for a branched plant

Transplant at the first true leaves, which are the second pair to appear and are shaped like the adult foliage rather than like the rounded seed leaves. Waiting longer means the roots of neighboring seedlings tangle and tear apart. Loosen underneath with a dibber or the end of a pencil, lift each seedling by holding one of its leaves, never by the stem, since a crushed stem is fatal and a torn leaf is not, and lower it into a hole in a plug or small pot deep enough that the seed leaves sit just above the compost.

Grow them on cooler and brighter than they germinated: around 65F (18C) and the best light you have. This is where most home raised impatiens are ruined. Warm, dim conditions produce a seedling with two inches (5 cm) of bare pale stem below the first leaves, and that stem never thickens or shortens, so the plant is floppy for the rest of its life. If the seedlings are stretching, the answer is more light and less heat, not more feed.

Feed at quarter strength weekly from about two weeks after transplanting seedlings, and pinch the growing tip out once the plant reaches three inches (8 cm). Two shoots break below the pinch within a week and you get a branched plant with twice the flowering points, at the cost of about ten days. On a plant that will not go outside for another month, that delay costs nothing.

Note your sowing date and the date the first true leaves appeared, then the date you planted out. The stretch between sowing and a plant big enough to go outside is around ten to twelve weeks, and one season of your own figures beats any packet instruction for planning the following year.

Hardening off and the first weeks outside

Young impatiens go outside later than almost anything else. Wait until nights are reliably above 55F (13C), which is typically two to three weeks after the last frost date rather than on it. A young plant set out into cold soil stops growing, sheds its lower leaves and is still behind in midsummer, whereas one held indoors an extra fortnight and planted into warm soil catches up and passes it within weeks.

Harden off over about two weeks, starting with an hour in a sheltered shaded spot and increasing daily, bringing them in overnight for the first week. The critical point is that impatiens are hardening to cool air and moving air, not to sun. Keep them shaded the whole way through. A seedling raised on a windowsill and put into direct sunlight scorches its leaves within a single afternoon, and those leaves do not recover.

Once planted, the first three to four weeks are the vulnerable window. Keep the soil evenly damp, since a young plant with a small root system collapses much faster than an established one and takes longer to come back. Put slug protection down on the day you plant, because soft young impatiens stems are grazed off at ground level overnight and a whole tray can disappear in one night. Hold off feeding for two or three weeks, then start at half strength every two weeks. Pinch once more when the plant is growing away strongly, and after that it looks after itself until the frost.