Answer: Firs generally don't mind sharing soil space with groundcovers such as spiderwort. Spiderwort will grow in sun or shade but it likes moist soil, which you may not find under a fir tree (firs have extensive root systems and their canopies divert rainwater so the ground beneath a fir can be quite dry). If you want to grow spiderwort, I'd recommend planting in near but not directly beneath your fir. If it ventures out and grows beneath the tree on its own, you're in luck. If it avoids growing beneath the tree, the soil is too dry to suit it.
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