Doug fir canopy plants
What will grow well in the sandy coastal soil beneath our doug firs? We live near Flora's Lake near the coast. Thank you! (Vinca??
Curry County Oregon
Vinca ( Periwinkle) is not recommended as it has the capability of forming dense mats that can escape yards and become invasive.
You don't say how big or mature or how closely spaced your Doug firs are. If they are mature/close-spaced, the area below them is probably shady and relatively dry. Doug Firs use a lot of water and have relatively shallow roots that will compete very effectively for water with any plants in the understory. This can limit what might be able to grow there to plants that like the shade and can handle drier conditions once established.
You could consider shrubs like Evergreen Huckleberry, Oceanspray, Salal, Cascade Oregon Grape, Indian Plum. Possibly Western Sword Fern. Pacific Bleeding Heart and many Hostas can grow in shady areas too but will need additional water during their growing season even after establishment.
Considering the relatively shallow Doug Fir root systems, you would have to be careful to not damage tree roots when planting shrubs especially. If you do plant any of these understory plants, keep in mind you will have to ensure they get enough water throughout the year to allow them to get established.
If you trimmed limbs up six to eight feet, it might allow some sunlight for understory plants. With part shade (filtered sunlight) conditions you could consider planting small azaleas, rhododendrons, vine maples, serviceberry, snowberry. Again, you would have to ensure they get enough water throughout the year to get established.
Here is a link to Oregon State University Extension publication "Gardening with Oregon Native Plants West of the Cascades" that could also give you some ideas.