Cold Stratification
HOW TO COLD STRATIFY SEEDS
Everyone wants plants. Now. But there are a small group of seeds that require a special treatment before they will even germinate in a climate that has temperatures below freezing. This process is called “Cold Stratification.” There are two ways to accomplish this.
Fall Planting
If time is not a problem, you can put them in the ground in fall and let them go through a winter. They will sprout next year, but not flower. The following year, having gone through two winters, they will return and bloom as expected.
If you are planting poppies direct sowing is recommended in late fall or early spring for zones 2-8
Create a "False Winter"
The second way to do this, if you want to save time, is to create a “false winter.” What this does is trick the seeds into thinking they have been in the ground for an entire year. All they need is to be in your refrigerator for at least 2 ½ months.
About 3 months before spring, place seeds in a plastic bag with a handful of slightly dampened, clean peat, paper towel, or a mix of clean peat and sand.
Seal and label the bag with seed name and date, then store in the refrigerator (not freezer) for at least 2½ mos. before planting in spring. (The cold period mimics a full winter’s cold.)
Once your seed has been treated, it’s ready to plant when spring arrives.
Varieties that need Cold Stratification
FOR ADVANCED SEED STARTERS ONLY: These seeds are trickier to handle. Many are tiny, and most require a special cold treatment or absolutely nothing will happen.
Acanthus (Bear's-Breeches), Aconitum (Monkshood), Alchemilla (Lady's Mantle), Alstroemeria (Peruvian Lily), Anemone (Windflower), Angelica, Arum, Aruncus (Goat's Beard), Asarum (Wild Ginger), Asclepias (Milkweed), Astrantia (Masterwort), Baptisia (False Indigo), Bergenia, Buddleia (Butterfly Bush), Caltha (Marsh Marigold), Caryopteris (Bluebeard), Chelone (Turtlehead), Chrysogonum (Golden Star), Cimicifuga (Bugbane), Clematis, Corydalis (Fumitory), Crambe (Seakale), Cyclamen, Delphinium, Dicentra (Bleedingheart), Dictamnus (Gas Plant), Dodecatheon (Shooting Star), Eremurus (Foxtail Lily), Eryngium (Sea Holly), Eupatorium (Boneset), Euphorbia (Spurge), Filipendula (Meadowsweet), Fuchsia, Gaura (Butterfly Gaura), Gentiana (Gentian), Geranium species (Cranesbill Geranium), Goniolimon (German Statice), Gunnera, Helianthemum (Rock Rose), Helianthus (Perennial Sunflower), Heliopsis (False Sunflower), Helleborus (Christmas & Lenten Rose), Heuchera hybrids (Fancy-leaf Coral Bells), Hibiscus (Hardy Hibiscus), Hypericum (St. John's-Wort), Iberis (Perennial Candytuft), Incarvillea (Hardy Gloxinia), Iris species, Jasione (Shepherd's Bit), Kirengeshoma (Waxbells), Knautia (Crimson Scabious), Lathyrus (Perennial Sweet Pea), Lavandula (Lavender), Lavatera (Tree Mallow), Leontopodium (Edelweiss), Lewisia, Ligularia, Lobelia, Lysimachia (Loosestrife), Macleaya (Plume Poppy), Mazus (Creeping Mazus), Mertensia (Virginia Bluebells), Myrrhis odorata (Sweet Cicely), Nepeta (Catmint), Oenothera (Evening Primrose, Sundrops), Omphalodes (Naval-seed), Penstemon (Beard-tongue), Perovskia (Russian Sage), Persicaria (Fleeceflower), Phlomis, Phlox (all types), Physalis (Chinese Lantern), Platycodon (Balloon Flower), Podophyllum (May Apple), Primula (Primrose, all types), Pulsatilla (Pasque-flower), Ranunculus (Buttercup), Ratibida (Prairie Coneflower), Rheum (Rhubarb), Rodgersia, Rosmarinus (Rosemary), Rudbeckia (Black-eyed Susan, most types), Rumex sanguineus (Bloody Dock), Sanguinaria (Bloodroot), Sanguisorba (Burnet), Saponaria (Soapwort), Saxifraga (Saxifrage), Scabiosa (Pincushion Flower), Sedum (Stonecrop), Sempervivum (Hen-and-Chicks), Sidalcea (Prairie Mallow), Silene (Campion, Catchfly), Sisyrinchium (Blue-eyed Grass), Stokesia (Stokes' Aster), Teucrium (Germander), Thalictrum (Meadow-rue), Tiarella (Foamflower), Tradescantia (Spiderwort), Tricyrtis (Toad-lily), Trollius (Globeflower), Vernonia (Ironweed), Veronica Dwarf Types (Speedwell), Veronicastrum (Culver's-root), Viola species types (Violets), Waldsteinia (Barren Strawberry), Zantedeschia (Calla Lily).