Answer: The roots of your clematis will eventually venture out into the native soil, no matter how much you amend so my suggestion is to amend only a small area by mixing compost with the soil you're going to dig out of the planting hole. Mix 50/50 compost and native soil, roughen up the edges of the planting hole, replace the soil into the hole and then dig out enough to plant your clematis. Amend a small area, only one foot by one foot. This will give the roots of your plant enough good soil to promote root growth. By the time the roots fill the area you've amended, they will be strong enough to venture out into the native soil.
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