Assisted tree migration: Stratification & Sowing

This blog post is part 2 of a series following the Tree Population Assisted Migration Project—an experimental planting designed by Oxbow’s Conservation Team, headed by Matt Distler, Ph.D., where we will monitor and track the growth and health of different tree species and tree populations from various geographic sources to understand which species and populations are better adapted to thrive long-term in the face of climate change. Click here to read Part 1, in which we talked about what exactly Tree Migration is, and outlined the project goals, map, and more. Click here to read Part 3, where we show why our planting is more pertinent than ever after record-breaking summer and fall weather.

Over the past few months, Oxbow’s Conservation team & volunteers have removed invasive species, such as Himalayan blackberries, from 1.2 acres of Snoqualmie River shoreline here at Oxbow. As a result, they have created a favorable environment for the future home of 3,000 new native trees and shrubs. While our talented team of conservationists continues to upkeep, maintain, and monitor the soil conditions where the new transplants will live, our Native Plant Nursery (NPN) team is taking the next steps to assist in this collaborative tree migration project. 

One of the eight species we will plant on this site is bigleaf maple (Acer macrophyllum). Some of the seeds come from here in the valley, while others come from central and southwest Oregon, where trees are adapted to different climate patterns than those experienced by our local trees, including warmer winters and drier summers.

Oxbow’s NPN has begun the process of stratifying and sowing the seeds, to help them build strong root systems to grow into healthy saplings that will thrive when transplanted into the site that the Conservation team has prepared for monitoring, testing, and documenting how each species adapts to changing climate conditions. 



Bigleaf maple seeds are typically sown in fall or soon after collection. However, dry seeds that have been stored for spring sowing require cold-wet stratification for 60 to 90 days (about 3 months) before sowing.  

Many native plant species have seed dormancy phases, and they will not germinate until this dormancy is broken. In the wild, these native seeds spend time in the ground during the winter, having their coats softened by frost and natural exposure to the weather and the elements. This cold and moist stage triggers the seed’s embryo, and its growth and expansion eventually break through the softened seed coat in search of sun, warmth, and nutrients.  

Stratification is a process in which we artificially simulate natural germination conditions. In early Feb 2022, our Native Plant Nursery team began the seed treatments necessary to give our seed friends the push they needed to sprout.  

For these seeds, Talinna Appling, Oxbow’s Native Plant Expert, soaked them in water for four days to ensure they were well hydrated before placing them in a soft mesh bag that allows for breathability and easy monitoring. The seeds within the bag were then buried in a bucket filled with moist peat moss and placed in our seed cooler at 38°F and periodically monitored for progress, conditions, and moisture levels. In late March 2022, the seeds were starting to sprout and were ready for sowing. 


Sowing & Next Steps 

Oxbow’s NPN uses open-bottomed plug containers that are easily recycled and allow the plants to grow a detangled, healthy root system and transplant easily when ready. When the seeds in the stratification process started sprouting, it was time to remove them from the cold and support them in their search for warmth, soil nutrients, water, and, eventually, sunlight.  

Each plug will host two seeds during this next stage to ensure that at least one of them will sprout and grow. Once we are sure germination is complete and we have the adequate numbers needed for the project, we will thin the seedlings so that there is only one per cell so that they can grow in the plugs without competition. The NPN will monitor them until they are ready to be transplanted on site in the fall. Oxbow’s Conservation & Native Plant Nursery teams will ensure they give these trees the best chance of success so we can continue monitoring their progress for years to come.

Once the seedlings are strong enough, they will be transplanted into larger pots to support their healthy development. By fall, they will be ready to be transplanted to their site. Pictured below, you can see where these saplings will make their future homes. You will notice squared parcels that delimit the areas for planting. Currently, you can observe small blue tubes protecting other young native plants that have been planted here to support the natural habitat that we are hoping to build in this restored riparian area. 


There is still a long journey ahead for these young trees. To follow this project and learn more about population-assisted tree migration, subscribe to our newsletter, or follow the hashtag #AssistedMigrationOxbow

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