Answer: Spring and fall are generally the best times to transplant trees and shrubs. Natural rainfall during these times, and cooler weather, reduce the stress a plant faces when it's uprooted and moved.
It sounds as though this Doug fir grew naturally rather than having been planted in your field. This presents some problems in digging and transplanting. Native plants and volunteer plants usually have extensive and deep root systems - that's how they're able to survive in the wild without summer watering and other general care. While the tree in question may be only 6' tall, it could have an amazingly large root mass. The only way you'll know is to start digging. If possible, soak the soil thoroughly the day before your planned move (to make digging easier). Start digging about 2' away from the trunk, carefully excavating so as not to damage the roots. Dig as deeply as possible, making sure you keep the rootmass intact. Then transplant as quickly as possible so the roots don't dry out. Place the tree at the same level it was growing before and water well. To ease the transition, water weekly until new growth appears.
Hope your Doug fir comes through with flying colors!
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