Growing Petunias from Seed: Surface Sowing, Transplanting seedlings and Pinching

Sow on the surface and never cover the seed

Petunia seed is among the smallest a home gardener handles, close to fine dust and easily lost on a windy sill or in the crease of a packet. It also needs light to germinate, which means that covering it with even a thin dusting of compost is enough to stop the batch entirely. Scatter it on the surface of firmed damp compost, press it down gently with a flat board so it makes contact, and leave it uncovered.

Sow far more thinly than feels sensible. A pinch of seed contains hundreds, and a crowded surface guarantees damping off later because the seedlings are too small to separate and air cannot move between them. Mixing the seed with a spoonful of dry silver sand before scattering makes the distribution visible and is the easiest way to sow evenly. Pelleted seed, where each seed is coated into a small ball, is much easier to space individually and is worth the extra cost for small quantities; it still must not be covered, and the coating needs to be kept damp to break down.

Water the tray from below by standing it in an inch of water until the surface darkens, then lift it out. A watering can from above washes surface sown seed into the corners of the tray and down into the compost, where it will not germinate.

1. Fill and firm first Use a fine seed-starting mix, level it and press it flat with a board to about half an inch (12 mm) below the rim. Sowing onto loose compost lets the seed drop into the gaps and out of the light.
2. Water the compost before sowing Soak from below and let it drain. Wetting after sowing is what moves the seed, so having the compost already damp removes the problem.
3. Mix the seed with dry sand and scatter A teaspoon of silver sand to a pinch of seed lets you see where the seed has gone and stops you emptying half the packet into one corner.
4. Press, do not cover Firm the surface lightly with the board so each seed touches damp compost. Any covering at all blocks the light the seed needs.
5. Cover the tray, not the seed A clear lid or a bag holds humidity until germination. Take it off the day the first seedlings show.

Warmth and light for germination

Hold the tray at 70 to 75F (21 to 24C) and expect germination in seven to ten days, sometimes stretching to two weeks. Below 65F (18C) germination is slow and uneven, and a tray on a cold windowsill can take three weeks and come up in patches. A heat mat makes the difference between a scattered result and an even one.

Bright light from the moment you sow, but not blazing direct sun through glass, which cooks a surface sown tray dry in an hour. A position under grow lights a few inches above the compost is ideal; a bright north facing sill will do; a warm dark airing cupboard, which works for many seeds, produces nothing at all from petunias.

The moment the first seedlings appear, take the cover off and drop the temperature to about 65F (18C). Leaving the lid on past germination is the single commonest way to lose a petunia tray, because still saturated air over congested seedlings is exactly the condition damping off needs.

The slow three weeks after germination

Petunia seedlings emerge tiny and then appear to stop. For roughly three weeks they sit as pinhead sized rosettes while everything else on the bench doubles in size. This is normal and it is not a sign that anything is wrong. The plant is building a root system before it builds a top, and in week four it visibly accelerates.

Do not respond by raising the temperature or by feeding heavily. Warmth without matching light produces long pale stems that never thicken, and a drawn petunia seedling stays a poor plant for the rest of the season. Light is the thing to add, not heat. If the seedlings are leaning, turn the tray daily, but a leaning tray usually means the light source is too far away rather than too much on one side.

Keep the compost just damp and never wet. Water from below, and let the surface dry slightly between waterings. A constantly wet surface with still air over it is where damping off starts, showing as seedlings toppling with a thin water soaked constriction at soil level. That patch will spread outward through the tray, so remove the affected seedlings and the compost around them at once and improve the airflow.

Note the sowing date and the date you first see green. After two seasons you will know your own germination time exactly, and that is what tells you whether a slow tray needs more patience or a fresh sowing.

Transplanting seedlings and potting up

Transplant when the first pair of true leaves has opened, which is usually two to three weeks after germination. The seedlings are small enough at this point that a plant label or a pencil tip is the right tool for lifting them. Hold a seed leaf between finger and thumb and never the stem, which is thin enough to crush without you feeling it, and a crushed stem is fatal even though the seedling looks fine for a day or two.

Move them into plugs or into two to three inch (5 to 7.5 cm) pots, one to a station, setting each seedling so its seed leaves sit just above the compost surface. Firm gently and water from below. Then drop the temperature to around 60F (16C) and give the brightest position you have. Cool and bright at this stage is what produces the short stocky plant that will fill a basket.

Start a half strength liquid feed about two weeks after transplanting seedlings, once the roots have taken hold, and repeat weekly. Pot up again into four inch (10 cm) pots before the roots circle the plug, because a petunia that goes hard and root bound in a small pot flowers early on a small plant and never builds bulk afterwards.

Pinching and hardening off before they go out

Pinch out the top half inch (12 mm) of the main shoot when the plant has four or five pairs of leaves. Two side shoots break below the cut, and pinching each of those once more when they have three or four pairs gives you eight growing points on a plant that would otherwise have run up as a single stem. This is what stops the plant going bare in the middle by midsummer, and it is worth the two weeks of delayed flowering it costs.

Harden off over ten to fourteen days before planting. Start with two hours in a sheltered shaded spot, increase the exposure daily, and finish with a full night outside before planting. Petunia foliage grown soft under cover scorches in a single sunny afternoon if it is put straight out, and a scorched plant is set back by weeks.

Do not plant out until the last frost has passed and nights hold above 50F (10C). Cold soil turns petunia foliage purple and stalls the plant, and one that has been checked in this way is slower to fill out than a plant put out a fortnight later into warm ground.

Keep a note of which seedlings you pinched and which you did not, alongside how each looked at midsummer. One season of that comparison settles the question of whether pinching is worth the delay far better than any general advice can.