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Seed Stratification

By Owl McCabe


Spring is a time for planting seeds. However, not all seeds can be treated the same. There are some seeds that need a period of cool weather for them to start. This is natures way to ensure that seeds do not start too early and then getting frozen. The process of seed starting for these types of seeds is called stratification.

Getting the correct amount of cooling and moisture can be a little difficult. I have tried different ways of stratification but not always with success. I would like to share what I have found that works more often than not.

First you need to know how long your seeds need to stay cool. Most tree seeds need about 3 months of stratification while some perennials need only one month. Counting backwards from when you want to have the seeds germinate, calculate the timing of the start of stratification. For example, I have some Canadian bush cherry seeds that I know need three months of stratification. Since I want them to be ready for transplanting outside by the first of June after having spent 10 weeks in a greenhouse after sprouting, I had to start stratification in late December. I also want by purple asters to be ready at the same time. They only need 1 month of stratification and only 8 weeks in the greenhouse so I would start the stratification early March.

The starting mix I use is one part vermiculite to two parts peat moss that has been sifted through ¼ inch hardware cloth. Before I plant any seeds, I soak this mix overnight in rainwater. I find using rainwater to be very important. Young seeds are intolerant of salts which can be found in well or city water. I use small 3 ½ inch pots filled with the potting mix. Usually I can fit all the seeds into one or two pots depending on the amount in a packet. I then cover the seeds with plain vermiculite to the depth recommended on the seed pack. I cover this with plastic wrap and secure with a rubber band or string and place these pots in a refrigerator. I make sure to mark them with the date they finish stratifying and need to come out.

Once stratification is done, I place the pots in my seeding chamber where they have constant heat and humidity. If you do not have a seeding chamber, I would recommend a seed starting heat mat. Once you see any sign that the seeds are sprouting, remove the plastic wrap immediately and ensure the starts have plenty of light. Once the starts have produced their first true leaves, they will need to be transplanted into individual pots. When I say true leaves, I am not talking about the first green leaves you see, the cotyledons, but the next set of leaves that look like the actual leaves the plant will have. Be very careful when transplanting these young sprouts, making sure not to touch the roots with bare hands. They do not like our body oils. As they grow, water as much as possible with rainwater. As they get older and are getting close to being transplanted outside, I would start using the water they will be getting on a regular basis.

Seed stratification sounds a little complicated but do not fret. If you follow these directions, you should be able to successfully start seeds that need stratification.


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