Milkweed seed stratification and vernalization

March 3, 2016 – Kenosha News

Monarch butterfly on milkweedQuestion: I ordered milkweed seeds to plant in my yard from a monarch protection group. The directions say the seeds need cold stratification. I looked online and a couple of monarch websites mentioned stratification/vernalization for milkweed seed. Would you explain what these words mean and what I should do to get my seeds to grow? N.M.

Answer: I can understand why you might be confused. Let’s start by clarifying horticulture terminology. The terms “stratification” and “vernalization” are not the same. Stratification relates to seeds and vernalization relates to flowering.

Stratification is a period of moist-chilling (also known as cold, moist) or moist-warm conditions that overcomes physiological dormancy requirements in seeds of some plant species. Cold, moist stratification is required for some Wisconsin native plants, like milkweeds (Asclepias), which release their seed in late summer or fall. The seed’s physiological dormancy prevents germination late in the growing season which would force a young seedling to survive winter. Instead, the seeds are dormant though much of winter, which naturally fulfills the moist-chilling requirement, and the seeds germinate in spring.

Vernalization is a period of cold temperatures required by some plant species to induce flowering. Many biennials – plants with a two year life cycle of seed to plant to flower to seed – and some perennials require vernalization. Examples of plants requiring vernalization are carrot and beard-tongue (Penstemon).

You may see one other horticultural term related to seed germination and that is scarification. In order to germinate, water and air must enter the seed. In some plant species the seed coat, which is the outermost layer of a seed, is impermeable to water and air. Scarification is breaking, scratching, or softening the seed coat so water and air can enter the seed. The easiest and safest way for hobby gardeners to scarify seed is rubbing on sandpaper or nicking with a file.

A good seed propagation reference will tell you which seeds need scarification and/or stratification.

Milkweed seeds don’t require stratification but the percentage of seeds that germinate is greatly increased by cold-moist stratification. The easiest way to stratify seeds is to mimic nature by sowing the seeds outdoors in fall. Seed stratification can also take place in your refrigerator. Mix your milkweed seeds with moist potting soil or sand and put into a zip-top plastic bag. Refrigerate for four to six weeks. Afterwards, sow seeds in three to four inch deep pots to reduce root disturbance when transplanting. Milkweed seeds need temperatures above 65 degrees to germinate.


Kenosha County UW-Extension LogoBarb Larson is horticulture educator for the Kenosha County University of Wisconsin Extension. She holds a master’s of science in horticulture from the University of Wisconsin-Madison. If you have a plant or gardening question, email her at or call 262-857-1942.