We might say that the body could be created, built of herbs. Blodwedd is an example of a living human being magically created from nine kinds of herbs. We also know from a Welsh medieval medical text, and from Irish tradition, that the body is related to the cosmos in Celtic thought. The eyes may be the stars, sun be the face, breath be the wind, stone as bones, water as blood, soil as flesh, etc. The sacrifice of plants would not be invalid as a sacrifice of spiritual efficacy.
Plants have sentient spirits, no less than humans or animals.
In fact, I would argue that plants are more important than humans or animals in a biological sense, for without plants, the energy of the sun cannot be transformed into food, and no life would exist without them. Plants can exist without us, but without them there is no possibility of independent animal life. The life force of a plant may, therefore, be considered just as pure and acceptable for sacrifice as would the life of a cow, or a human being. From the point of view of primacy, it might even be considered more acceptable, more pure, as they are closer to the primal source of solar energy than we. Individual plants lack only the social construct of the complete human body to be an acceptable substitute for the human being.