7 Step Sweet Pea Sowing Routine with Spencer Mix in Rootrainers During January

Spencer mix sweet peas are the frilled, large-flowered type bred from Countess Spencer around 1900. They have waved standards and long stems, qualities that make florists rate them above the older grandifloras. Sown in January, the seedlings spend eight to ten weeks under low light making root mass, and by March planting they are ready to push flowering stems of 30 to 40 centimetres. Rootrainers are Haxnicks book-fold modules, hinged so the cells open flat, and each seedling sits in a 12 centimetre deep chamber with vertical ribs that keep the taproot from coiling at the base.

Step 1: Chit the dark seed first

Seed coat colour changes the treatment. Beaujolais and other dark-seeded varieties have a hard testa that can be slow to take up water. Pale cream seeds, including Mrs Collier, soften quickly and can rot if they sit on damp paper before sowing.

Put the dark seeds on damp kitchen paper in a lidded margarine tub. Keep the tub at about 15C on a windowsill away from direct sun, then look at them after 48 hours. Once the radicle appears as a white nub one or two millimetres long, the seed has chitted and can go into compost.

The pale seeds go straight into the filled cells and normally germinate within a week. If you prefer to nick the dark seed coats, cut on the side opposite the eye with a craft knife, although small seed can lose too much testa and the embryo may dry. Paper chitting is safer because only seed that has already shown life is sown, and a packet of 25 typically gives 20 or more viable seeds.

Step 2: Fill the Rootrainer books firmly

Sieve peat-free multipurpose compost through a 6 millimetre riddle so coarse bark does not jam the narrow cells, then mix in about 20 percent horticultural grit by volume to help the compost column drain and keep the root out of wet compost. Stand the eight-cell Rootrainer books in their rack, fill every cell proud of the rim, tap the rack twice on the bench, top up, and firm lightly with a thumb. Loose pockets in the compost make the descending root bridge a gap, which leaves it kinked.

Step 3: Sow at 2 centimetres and water from below

Make a 2 centimetre hole in the centre of each cell with a dibber or pencil. Set one chitted seed in the hole with the radicle pointing down, following the direction it has already started, and cover it with sieved compost.

That 2 centimetre depth is a useful working mark. Seed set too close to the surface can heave as it swells. If it is buried much deeper, the cotyledons spend their reserves before they reach the light.

Water the whole rack from below by standing it in a tray of water until the surface darkens. With dry peat-free mix this takes about ten minutes, after which the rack should be lifted out to drain. Overhead watering at sowing time can cap the surface, leaving the new shoot to force its way through a crust. Label each book in pencil with the variety and sowing date, since Spencer seedlings look alike and the colours separate only when they flower in June. The January date also marks when the ten-week hardening-off window is approaching.

Step 4: Keep germination cool

Sweet pea seed germinates best at 12 to 15C. An unheated porch, a cool spare room, or a cold greenhouse with one sheet of fleece can provide that range in January. A heated propagator at 21C brings on leggy, drawn seedlings that flop.

At 12 to 15C, chitted seed usually throws a shoot in five to eight days. Pale seed sown dry takes a few days longer. Keep the compost just moist and avoid saturation. As soon as green appears, put the rack in the brightest cold position available so the first internode stays short.

Step 5: Pinch when true leaves have formed

Wait until each seedling has two or three pairs of true leaves above the cotyledons. That stage usually arrives three to four weeks after germination. Use finger and thumb, or a clean snip from Niwaki Okatsune shears, to remove the growing tip.

Taking out the apical tip breaks apical dominance. The dormant buds in the lower leaf axils then begin to grow. After a single pinch, expect two to four basal shoots from each seedling. A plant left with its tip often races upward as one weak stem, flowers earlier, and gives shorter stems. The basal shoots are the later cordons, so the pinch increases the number of cutting stems available from the plant.

Pinch once. Repeating the cut holds flowering back by another fortnight and adds no useful shoots. With a January sowing, that delay can move the first flowers past the period when they stand best in the cutting border.

Step 6: Use the cold frame before planting out

Move the Rootrainers into a cold frame in late February, once the seedlings carry their basal shoots. The frame softens the jump between a 2C night and a 12C afternoon, a swing that would check young plants standing unprotected in open ground.

On mild days, prop the light open a hand’s width. Close it by mid-afternoon so some warmth remains inside, and shut it fully when frost threatens. The process takes ten to fourteen days. During that spell the stems gain substance, the leaves become waxier, and new growth sits closer to the previous node instead of stretching.

On the coldest nights, a sheet of horticultural fleece over the closed frame adds two or three degrees of protection. Sweet peas can take light frost once hardened, so the cold frame is there to prepare them for outside weather while keeping growth moving.

By late February, the taproot has usually reached the bottom of the 12 centimetre cell and fibrous roots have filled the ribs. Open the hinged book and the plug lifts out whole, with the root system undisturbed, which is the point of the Rootrainer design. Seedlings raised in shallow trays often tear at transplanting and then sulk for a fortnight.

Step 7: Plant the plugs and choose the cordon

Plant the intact plugs 20 centimetres apart. A wigwam works for garden display. For show-length stems, use a row of vertical canes or strings tied to a raspberry post and wire system that has been borrowed for the season.

Set each plug slightly deeper than it stood in the cell, allowing the lower stem to root into the soil. After planting, lay a 5 centimetre band of Strulch mineral mulch around the base. The mulch holds moisture, and the iron in the product deters slugs from the soft new growth.

For cordon training, keep the strongest basal shoot on each plant and remove the others at the base. Tie the retained shoot to its support every 20 centimetres of growth. Through May and June, pinch out each tendril and side shoot as soon as it appears. Removing tendrils stops the plant grabbing neighbouring stems, and the growth goes into stem length and flower size.

A cordon-trained Spencer can make well over a metre of clean stem, with flowers spaced for cutting. Left to grow naturally, the same plant becomes a tangled bush.

Seedlings from the same Spencer mix can sit in a cold frame with identical green leaves and no hint of which colours deserve the space. Until the first buds show colour, the pencilled label carries the only difference visible above the compost.