Cold Stratification: A Winter Prep for Growing Cut Flowers at Petals Flower Farm

Snow on crabapple tree blossoms

As flower farmers here at Petals Flower Farm in Boulder, Colorado, we know all too well that our growing season is a bit of a race. With our Zone 5b climate—where winter temperatures regularly dip below freezing and the growing season is shorter than in warmer regions—it’s crucial to maximize every moment of sunlight and warmth when spring finally arrives.

One of the tricks we’ve learned over the years to make sure our flowers have the best shot at thriving is cold stratification.

But what exactly does that mean? And how does it help flowers bloom brighter and stronger? Let’s dive into the world of cold stratification and explore how we use it to get the best start for our flower crops each season.

What is Cold Stratification?

Cold stratification is a process that mimics winter conditions for seeds that need a cold period to break dormancy. Many flower seeds, particularly perennials and certain hardy annuals, have evolved to need a period of cold before they can germinate.

This cold period helps to “wake up” the seeds, allowing them to sprout when the warmer spring temperatures arrive.

In essence, it's a way of tricking seeds into thinking they've gone through the winter, so they’re ready to grow when the time is right. At Petals Flower Farm, we use cold stratification for a number of our most beloved flowers—especially those that are native or hardy enough to handle our chilly winters. By giving seeds the cold treatment they need, we ensure that when planting season rolls around, we’re ready for a full, healthy bloom.

How to Cold Stratify Seeds

The good news is, cold stratification is a very simple process! Here’s how we do it at Petals Flower Farm, so you can try it too:

1. Choose the Right Seeds: Not all flower seeds need cold stratification, so it’s important to know which varieties benefit from this process. We'll get into that in a bit, but first, let’s outline the general steps.

   
   

2. Hydrate the Seeds: All flower seeds benefit from hydration. This softens the seed coat and helps moisture reach the embryo inside, making it easier for the seed to break dormancy once it’s exposed to cold. Simply wet a square of paper towel and lay it out flat on your countertop.

3. Spread the Seeds: Sprinkle the seeds out on the paper towel. I try to spread them out in the center so they aren't all in a pile on top of each other but I'm not finicky about making sure they are evenly spread. Fold the paper towel over the seeds so they are neat and covered and the square of damp towel can fit into a ziplock. For most seeds I use a snack size ziplock bag, for some of the larger or more numerous seeds I use a sandwich sized bag. Be sure to label your bags so you know which seeds are which.

   
   

4. Refrigerate the Seeds: After preparing the seeds in the moist paper towel, it’s time to place them in the refrigerator. This is where the cold stratification magic happens! I place my bags standing upright in a glass Tupperware container with no lid. Place the bag or container in the fridge (not the freezer—freezing temperatures are too extreme for most seeds). The cold temperatures will trigger the germination process, mimicking the natural winter conditions. Depending on the seed variety, the cold stratification period can range from two weeks to 90 days. For simplicity's sake we cold stratify all of our seeds for 14-21 days, pulling them out whenever we are ready to pot the seeds up.

   
   

5. Plant the Seeds: Once the cold stratification period is over, the seeds are ready to be planted. You’ll notice that the seeds may or may not look a little different—some may have swelled slightly, or you might even see tiny rootlets starting to form. This is a good sign that the seeds are primed for germination. Do not worry if they look exactly the same as they did when you nestled them into their cold blankets.

   
   

   At Petals we use this seed starting mix that we make in house. Its easy and saves money over pre-made mixes.

   
   

Which Flower Seeds Benefit from Cold Stratification?

At Petals Flower Farm, we rely on cold stratification for several of our most popular flower varieties. Here are some of the best flower seeds that thrive with cold stratification:

• Larkspur (Delphinium)Larkspur seeds are one of our favorites, as they produce tall, elegant spikes of blooms. They benefit greatly from cold stratification and require about 4-6 weeks of cold exposure.

• Shasta Daisy (Leucanthemum)These cheerful white flowers with yellow centers are a staple in our summer bouquets. Cold stratification is essential for getting them to germinate, with a typical cold period of 30-60 days.

• Columbine (Aquilegia)Columbines are native to Colorado and do particularly well in our mountainous environment. Their seeds need cold stratification for about 30 days to help ensure good germination rates.

• Poppy (Papaver somniferum)We love the bright, delicate blooms of poppies. These seeds also benefit from cold stratification, which helps them break dormancy and germinate effectively. A 30-day cold period is generally sufficient for poppies.

• Coneflower (Echinacea)Known for their daisy-like flowers with bold purple, pink, or white petals, coneflowers are another perennial that does well with cold stratification. They require about 60 days of cold exposure.

• Milkweed (Asclepias)A critical plant for pollinators, particularly Monarch butterflies, milkweed seeds need a cold period of around 30-60 days to break dormancy and sprout properly.

• Aster (Symphyotrichum)These late-blooming flowers are a lovely addition to fall bouquets. Cold stratification helps aster seeds to germinate and produce beautiful blooms after the cooler months.

• Snapdragons, Foxglove, Bells of Ireland, Hollyhocks, Verbena (!seriously, germination is 1000% better with cold stratified verbena)

  
  

Why Cold Stratification Matters for Cut Flower Gardens

In our urban flower farm here in Boulder, where we work hard to grow a diverse mix of flowers for our local community, cold stratification is a game-changer. By giving these flowers the cold treatment they need before starting them indoors or even sowing them directly in the garden, we’re ensuring that they get a head start in the growing season, making it more likely that we’ll have a bountiful harvest to share with our customers.

By practicing cold stratification, we’re not just helping our flowers grow, but also supporting the ecosystems around us. Many of the flowers that benefit from cold stratification—like milkweed, columbine, and coneflower—are vital for pollinators like bees, butterflies, and hummingbirds.

These flowers are more than just beautiful—they’re essential to a healthy garden and a healthy planet.

So, if you're a flower farmer—or even just a passionate home gardener—embracing cold stratification is a simple yet powerful way to enhance your flower production. With a little patience and preparation, you’ll have healthy, vibrant flowers ready to bloom as soon as the warm weather arrives.

Happy planting from Petals Flower Farm! 🌸

Getting You Blooms by Barefoot Season