How to Sow Dianthus Seed: Temperature, Depth and Growing On

What seed will and will not give you

Dianthus is one of the easier things to raise from seed. Germination is quick and reliable at moderate temperatures, the seedlings are tough, and a single packet gives you enough plants to fill a wall top or an edge of a bed for the cost of a single pot plant. If you want numbers, or you want to fill a difficult dry sunny spot cheaply, sowing is the sensible route.

What seed will not do is reproduce a particular plant. Seedlings segregate for flower color, form, habit and, most importantly, scent. Raise twenty plants from a strongly clove-scented parent and a good proportion of them will smell of nothing at all, and there is no way to tell which until they flower. This is why anyone who values a fragrant plant keeps it going by cuttings and treats seed as a separate exercise.

Approached in that spirit, seed is worth sowing every year or two. You get variation, occasional plants better than anything you bought, and a supply of replacements for the ones that inevitably die of winter wet. Judge the seedlings at midday in warm weather in their first flowering season, keep the ones that smell, and take cuttings from those.

Temperature, depth and timing

Sow at 60 to 70F (15 to 21C) and expect emergence in seven to fourteen days. The seed is not fussy about warmth in the way that tender annuals are, and germination is actually more reliable on a cool bright bench than in a heated propagator, where the seedlings stretch and damp off. If you have a choice between too warm and too cool, choose too cool.

Depth matters more than temperature. The seed is small and should be covered by no more than an eighth of an inch (3 mm) of sifted compost or vermiculite, just enough to hide it. Seed pushed half an inch deep into wet compost will rot rather than emerge, and this is the commonest reason for a tray that never comes up. Firm the surface before sowing so the seed sits level rather than washing into hollows.

Sow in spring under cover for plants that will make a good clump by autumn and flower fully the following season. Where summers are long, a second sowing in late summer works too, giving small plants that overwinter and flower early. Direct sowing outdoors is possible once the soil is above 55F (13C), but the seedlings compete poorly with weeds at that stage, so a tray is easier to manage.

Growing the seedlings on without losing them

The danger period is between emergence and transplanting seedlings, and the danger is damping off. Seedlings collapse at soil level in a batch, usually where the compost surface has stayed wet and the air has been still and warm. Use a gritty, free-draining seed mix rather than a rich one, water from below where you can, give the tray moving air as soon as the seedlings are up, and drop the temperature rather than raising it.

Move the tray into the brightest, coolest place you have the moment the first seedlings break the surface. A day or two of dim warmth at this point stretches the stems permanently and no amount of later light will thicken them again. Around 50 to 60F (10 to 16C) in full light produces short, stocky plants with the gray cast that tells you they are growing properly.

Transplant at the two-true-leaf stage into individual small pots of gritty potting mix, handling each seedling by a seed leaf rather than by the stem, since a crushed stem never recovers. Grow them on outdoors in the open once the weather allows, harden them off over ten to fourteen days, and plant out after the last frost with the crown level with the soil surface, never below it.

1. Fill and firm a gritty mix Use seed-starting mix with added perlite or sharp sand. Fill the tray, tap it down, and firm the surface level so the seed will not wash into hollows when you water.
2. Sow thinly and cover barely Space the seed about half an inch (1 cm) apart and sift the thinnest layer of vermiculite over the top, no more than an eighth of an inch (3 mm). Water in from below.
3. Hold at 60 to 70F (15 to 21C) A bright unheated windowsill suits better than a propagator. Expect the first seedlings within seven to fourteen days; germination is usually even.
4. Cool and brighten at emergence Move the tray to the brightest, coolest position available, ideally 50 to 60F (10 to 16C), and give the seedlings air movement. This prevents both stretching and damping off.
5. Transplant at two true leaves Lift each seedling by a leaf with a dibber and pot it into gritty potting mix. Set it at the depth it was growing; burying the collar starts rot early.
6. Harden off and plant out Stand the pots outside by day for ten to fourteen days before planting into a sunny, sharply drained spot, six to twelve inches (15 to 30 cm) apart with the crown at soil level.

Do not enrich the bed before planting seedlings out. Compost and general fertilizer produce soft, leafy plants that flower poorly and rot over their first winter. Grit and a scattering of lime are the right preparation.

Selecting from your seedlings

Seed raising only pays off if you are willing to throw plants away. In their first flowering season, go along the row at midday on a warm still day and smell every plant. Scent is at its strongest then and almost undetectable in cold or breezy weather, which is why a quick check on the wrong morning gives a misleading answer. Mark the ones that smell strongly, note their color and habit, and be honest about the rest.

Keep the good ones, take cuttings from them that same summer so the plant is secured, and remove the anonymous ones to make space. A bed of twenty seedlings will typically yield three or four genuinely worth keeping, and those few, propagated by cuttings, will still be with you in ten years.

What to record from a sowing

Note the sowing date, the date of first emergence and the temperature you held. Those three numbers, kept for two or three seasons, tell you what your own conditions actually deliver, which is far more useful than a general figure, and they immediately identify a bad batch of seed: no emergence within fourteen days at the right temperature is a seed problem, not a technique problem.

Then record where each seedling was planted and, once it flowers, its color and its scent judged at midday. Without a position record you cannot connect the plant you liked in its first summer to the mat you are looking at two years later, and the whole point of raising from seed, which is finding the few good ones, is lost.