Time to Give Hardy Seeds Their Cold Treatment

Many seeds require a lengthy period of cold temperatures before they will germinate. This group includes most trees and shrubs from cold and temperate climates, but also many perennials and even a few annuals. This process is called “cold stratification” because originally the seeds were layered (stratified) between layers of moist soil before exposing them to cold, but term “vernalization” is also used. The term “cold treatment” seems to be gaining ground, though, and it really does best explain the process.

The most obvious way of giving seeds a cold treatment is to sow them outside in the fall, but that often leaves them exposed to insects, mammals and inclement weather. It’s far safer to give seeds their cold treatment indoors, where you can keep a closer eye on them… and where nothing can eat them. It’s usually carried out in January or February so the young plants will be ready to plant out come spring.

Giving hardy seeds a cold treatment replicates what happens in the wild. There the seeds fall to the ground in the autumn and remain there all winter, exposed to cold and moisture. Then they germinate in the spring when the weather warms up. And many seeds require this cold-to-warm cycle: without it, they won’t germinate or will do so only very poorly.

The need for cold stratification developed over many millennia as a way of preventing seeds from germinating at the wrong season. Seeds that don’t need cold stratification often start to germinate when the weather is unusually warm in the late fall or when there is a January thaw, then the fragile seedlings are killed when cold weather returns. Those that have an obligatory need for stratification, however, won’t react to unseasonal conditions. They essentially have an internal clock telling them: “Look, it’s too early to germinate! Wait a few months more before you start to sprout.” In general, the longer the winters are in the plant’s native land, the longer the cold treatment it will require.

Cold and Moist

Beginners often don’t understand a vital detail: it’s not cold alone that stimulates germination, but cold combined with moisture. So you can’t just place the seed packets in a fridge for a few months and expect the seeds to germinate well, you need to put them into contact with moist soil first.

The other common error is freezing the seeds. Although most of these seeds will tolerate freezing temperatures, freezing the seeds is not necessary and actually slows the process down. For best results, give temperatures just above freezing, between 34°F and 41°F (1°C and 5°C). And as luck would have it, the temperature of a typical domestic refrigerator typical falls right in that range: about 35°F to 40°F (1.6°C to 4.4°C).

Traditionally, you sow the seeds that require a cold treatment in pot or tray, seal it inside a transparent plastic bag and place the container in the refrigerator. Just how long the cold treatment has to last depends on the species, from as little as 1 or 2 weeks to 4 months or more. Ideally you’d check the seed packet label for information. In case of doubt (for example, if you harvested seed yourself and have no idea of its needs), try three months. All these seeds require a minimum number of weeks in the cold, but there is no maximum. So no harm comes from prolonging the cold treatment beyond the minimum.

At the end of the cold treatment, remove the containers from the fridge and place them in a warm bright spot (about 21 to 24?C is ideal for most seeds) to stimulate germination. Many of these seeds are fairly slow to germinate, so don’t be surprised if they take 3 or 4 weeks, sometimes even longer.

From this stage on, simply treat the seedlings like any other. After germination, remove the plastic bag. Start watering whenever the soil starts to dry out. Fertilize when the plants have about four to five true leaves, etc. Finally, when the weather outdoors warms up enough, start acclimating the seedlings to outdoor conditions (place them in the shade for two or three days, then in partial shade for two or three days, then in the sun for two or three days). Once they’re well acclimated, transplant them either to a nursery (plants, such as trees, shrubs and slow-growing perennials, that will take more than a year to be presentable) or directly to their final location (annuals and fast-growing perennials).

If You Lack Fridge Space

If you lack space in your fridge, you can try a different method of cold stratification. Simply mix the seed in a few spoonfuls of moist vermiculite. Seal the vermiculite in a small plastic bag and put it in the fridge (this takes much less space and you can even pile your bags of seeds one on top of the other). When their cold period is up, simply lay the seed bags somewhere warm and fairly bright. As soon as you see little sprouts start to appear, very carefully pot up each seedling in its own little pot and water. Then proceed from there as above, growing them on and eventually planting them out.

Double Cold Stratification

For some seeds, a single cold treatment is not enough. It takes two! These seeds are very slow to germinate in the wild, often taking 2 or 3 years before they show any sign of life. However, you can get them germinate the very first year treating them to a double cold stratification. Here’s how:

Give the seeds 2 to 3 months in the fridge, expose them to warmth for 2 months, then put them back in the refrigerator for 2-3 months. This time, when you bring them out of the fridge, they should start to germinate… and if they don’t? Put them back in the fridge and try again. It once took me 4 alternating cold and warm treatments to get some stubborn trillium seeds to sprout!

Species Requiring a Cold Treatment

Here is a partial list of the seeds that normally require a cold treatment to germinate. However, there are many others. Always read the instructions on the back of the seed packet to see if the seeds you bought need this kind of care… or check out their needs on the Internet.

If in doubt, find out where the plant grows in the wild. If comes from a cold region and its seeds ripen in the fall, there is a very good chance that its seeds will require a cold treatment to germinate.

1. Althaea (marshmallow)
2. Astrantia (masterwort)
3. Baptisia (false indigo)
4. Buddleia (butterfly bush)
5. Caltha (marsh marigold)
6. Caryopteris (bluebeard)
7. Cercis canadensis (redbud)
8. Chelone (turtlehead)
9. Cimicifuga (bugbane)
10. Clematis (clematis)
11. Cornus (dogwood)
12. Corydalis (fumitory)
13. Delphinium (delphinium)
14. Dicentra spectabilis, now Lamprocapnos spectabilis (bleeding heart)
15. Dictamnus (gas plant)
16. Dodecatheon (shooting star)
17. Echinacea (purple coneflower)
18. Eremurus (foxtail lily)
19. Filipendula (meadowsweet)
20. Eryngium (sea holly)
21. Eupatorium (Joe Pye weed)
22. Forsythia (forsythia)
23. Fragaria (strawberry)
24. Fuchsia (fuchsia)
25. Gentiana (gentian)
26. Geranium (cranesbill)
27. Goniolimon (German statice)
28. Helianthemum (rock rose)
29. Helianthus (perennial sunflower)
30. Heliopsis (false sunflower)
31. Helleborus (Christmas rose)
32. Hemerocallis (daylily)
33. Heuchera (coral bells)
34. Hibiscus moscheutos (perennial hibiscus)
35. Hypericum (St. John’s wort)
36. Iberis (perennial candytuft)
37. Ilex* (holly)
38. Incarvillea (hardy gloxinia)
39. Kirengeshoma (waxbells)
40. Knautia (knautia)
41. Lathyrus (perennial sweet pea)
42. Lavandula (lavender)
43. Leontopodium (edelweiss)
44. Iris (iris, many species)
45. Lobelia (hardy lobelia)
46. Lonicera (honeysuckle)
47. Macleaya (plume poppy)
48. Magnolia* (magnolia)
49. Malus (apple, crabapple)
50. Mazus (creeping mazus)
51. Mertensia (virginia bluebells)
52. Myrrhis odorata (sweet cicely)
53. Nepeta (catmint)
54. Oenothera (evening Primrose)
55. Muscari (grape hyacinth)
56. Opuntia* (beavertail cactus)
57. Paeonia* (pivoine)
58. Penstemon (beard-tongue)
59. Persicaria (fleeceflower)
60. Phlox (phlox)
61. Physalis (chinese lantern)
62. Persicaria orientalis, syn. Polygonum orientale (kiss-me-over-the-garden-gate)
63. Picea (spruce)
64. Platycodon (balloon flower)
65. Primula (primrose)
66. Pulsatilla (pasque-flower)
67. Quercus (red and black oaks)
68. Ranunculus (buttercup)
69. Ratibida (prairie coneflower)
70. Rosa (rose)
71. Rudbeckia (black-eyed Susan)
72. Sambucus (elderberry)
73. Sanguinaria (bloodroot)
74. Sanguisorba (burnet)
75. Saponaria (soapwort)
76. Saxifraga (saxifrage)
77. Scabiosa (pincushion flower)
78. Sedum (stonecrop)
79. Sempervivum (houseleek)
80. Sidalcea (prairie mallow)
81. Silphium (cup plant, compass plant)
82. Staphylea* (bladdernut)
83. Stokesia (Stokes’ aster)
84. Syringa (lilac)
85. Thalictrum (meadow-rue)
86. Tiarella (foamflower)
87. Tricyrtis (toad-lily)
88. Trillium* (trillium)
89. Trollius (globeflower)
90. Tsuga (hemlock)
91. Vernonia (ironweed)
92. Veronica (speedwell)
93. Viola (violets)
94. Vitis (grape, some species)

*Some species in this genus require a double cold stratification.