Many native plants—including wildflowers, trees, and grasses—require a period of cold before they will germinate. This process, called cold stratification, mimics natural winter conditions that break seed dormancy. Without it, many seeds will not sprout, leading to frustration for beginning farmers and gardeners.

For Indiana farmers in USDA Hardiness Zones 5b to 6b, understanding how to stratify seeds is key to successfully growing native plants that support pollinators, improve soil health, and enhance biodiversity.

Why Cold Stratification Matters in Indiana

Indiana experiences cold winters and warm, humid summers, creating conditions where many native plant seeds naturally undergo stratification. In the wild, seeds drop in the fall, spend the winter buried in soil, and germinate when temperatures rise in the spring. When growing native plants from seed, replicating this process ensures strong germination rates.

Some common Indiana native plants that require cold stratification include:

• Wildflowers: Purple Coneflower (Echinacea purpurea), Butterfly Weed (Asclepias tuberosa), Black-eyed Susan (Rudbeckia hirta)
• Grasses: Big Bluestem (Andropogon gerardii), Little Bluestem (Schizachyrium scoparium), Indian Grass (Sorghastrum nutans)
• Trees & Shrubs: Pawpaw (Asimina triloba), Redbud (Cercis canadensis), Elderberry (Sambucus canadensis)

For farmers working to establish pollinator-friendly plantings, wildlife habitat, or native pastures, cold stratification is an important tool for successful seed germination.

Methods for Cold Stratification

There are several ways to stratify seeds, depending on your goals and planting schedule.

Refrigeration Method

For small-scale seed starting, refrigeration is an effective way to break dormancy.

1. Place seeds in a moist paper towel or damp sand inside a sealed plastic bag or container.
2. Store in the refrigerator for four to eight weeks, depending on the species.
3. Remove seeds and plant as usual when stratification is complete.

This method allows for controlled germination timing and is ideal for farmers planning nursery starts or transplanting seedlings in the spring.

Outdoor Winter Sowing (Milk Jug Method)

For a more natural approach, the milk jug method creates a miniature greenhouse effect.

1. Cut a clear plastic milk or water jug in half, poke drainage holes in the bottom, and fill it with soil.
2. Plant seeds and water, then tape the jug back together, leaving the cap off for ventilation.
3. Set the jugs outside in winter (December–February in Indiana), where they will experience natural freeze-thaw cycles.
4. When temperatures rise in spring, seeds will germinate at the right time.

This method requires minimal effort and works well for native wildflowers and perennials.

Direct Fall Planting

For large-scale plantings, sowing seeds directly into the soil in fall allows them to undergo stratification naturally. This works best for native grasses, meadows, and hedgerows. However, unpredictable winter conditions may affect germination rates, making controlled stratification methods preferable for more delicate species.

For farmers interested in native plant propagation, pollinator support, or conservation plantings, cold stratification is essential. Native plants are well-adapted to Indiana’s climate and provide long-term benefits such as:

• Improved soil health by reducing erosion and adding organic matter
• Increased farm biodiversity, supporting beneficial insects and wildlife
• Better drought tolerance, requiring fewer inputs than non-native species
• Potential market opportunities for those interested in selling native plants or seeds

Free Cold Stratification Workshop – February 22, 2025

For those interested in learning hands-on stratification techniques, a free workshop will be held at the IU Campus Farm in Bloomington.

Date: Saturday, February 22, 2025
Time: 2:00 – 4:00 PM
Location: IU Campus Farm, 451 N Pete Ellis Dr, Bloomington, IN 47408

Biologist Mary Welz from the Indiana Pollinator Partnership will demonstrate the milk jug method and discuss best practices for growing native plants.

What to Bring: A clean, empty ½ to 1-gallon plastic milk or water jug. Additional materials, including soil, will be provided.

RSVP by February 17th to receive further details.

For questions or directions, contact Christina at (765) 621-2924.

This workshop is a great opportunity for beginning farmers to practice cold stratification techniques, connect with experts, and prepare for a successful growing season.

For Indiana farmers looking to establish native plantings, improve pollinator habitat, or enhance biodiversity, cold stratification is a valuable technique. Whether using refrigeration, winter sowing, or direct fall planting, understanding how to break seed dormancy will lead to higher germination rates and stronger plant growth.

Attending the Cold Stratification Workshop is an excellent way to get started, meet fellow growers, and learn from experienced conservationists. We hope to see you there!