Starting Cut Flowers in Soil Blocks

Most cut-flower seed is small, quick, and sown by the hundreds — which is exactly what the 0.9″ block is for. Professional flower growers start nearly everything at this density, potting up only the slow and the sprawling. Here’s the size logic, then the flowers that ask for special treatment.

We started this company on a flower farm — the first blocks we ever pressed were for our own zinnias and dahlia cuttings — so this is the page we’d hand a friend who’s just ordered her spring seed.

What size block do cut flowers want?

The 0.9″ block is the flower farm workhorse: zinnias, snapdragons, celosia, marigolds, feverfew, basil for bouquets — experienced growers start the overwhelming majority of their seed at this density, hundreds to a tray. It dries fastest, so it asks for daily attention, and it means a Nano 25, Mini 75, or Commercial 200, depending on your volume. Large-seeded flowers — sweet peas, sunflowers, nasturtiums — skip straight to a 1.5″–2.25″ block (Nano 9 or Nano 4, Commercial 50).

The seed-starting fundamentals, blocked

The rules flower farmers already trust translate directly to blocks: sow at about twice the seed’s longest dimension; cover with a light dusting of fine vermiculite; bottom heat around 70°F until germination, then off; dome on until half to three-quarters of the tray is up, then off; lights two to three inches above the tops. And don’t mix flower types in one tray — germination rates differ, and the dome comes off at the wrong time for someone.

Flower by flower

• Sweet peas. Pre-sprout between damp paper towels in a lidded container somewhere warm; sow the sprouted seeds into 2″+ blocks. They germinate cool — around 55–65°F — and notoriously unevenly, so give them their own tray.
• Zinnias, cosmos, celosia. The easy ones: warm blocks (mid-70s°F), fast germination, and a fresh sowing every two to three weeks for continuous buckets. Don’t start zinnias more than four to six weeks before last frost — they punish eagerness by going root-bound.
• Snapdragons. The seed needs light — surface-sow, no cover beyond a whisper of vermiculite. And pack blocks firmly: a loosely packed block lets a snapdragon’s rootball shear apart at transplant.
• Dahlias from seed. Sow six to eight weeks before last frost at 70–75°F; seed-grown plants bloom the first year and form tubers you can lift in fall. For dahlias from cuttings, the oversized cells of the Dahlia Blocker are what we built it for — it’s named honestly.
• Lisianthus. The honest entry. It’s slow — start ten to twelve weeks early — it must stay below 75°F or it stalls into a rosette, and the seed is too fine for the paper-towel trick (the root binds into the fibers). Germinate it on moist mix in a shallow container instead, then lift each sprout with tweezers into a block’s dimple. Many growers simply buy plugs; if you love a challenge, this is the way.
• Ranunculus and anemones grow from corms, not seed — pre-sprout them cool and skip the blocker entirely.

Cool flowers vs. warm: two calendars

Hardy annuals — snapdragons, larkspur, sweet peas, bells of Ireland — germinate cool and can go out weeks before your last frost (or the previous fall). Warm annuals — zinnias, cosmos, celosia — want a heat mat and won’t forgive a frost. Sort your seed box into those two piles and the timing page does the rest.