In BC, I assume you are referring to western red cedar, Thuja plicata. Lots of plants will grow under the canopy of large conifers, providing there is at least some light and water available to get established. Some notable examples are sarcococca, Euphorbia robbiae, Geranium macrorrhizum, epimediums, salal, mahonia, lamium, fernleaf dicentra and various native ferns. Start with small plants (easier to dig holes through rooty soils), and water frequently to get established and fertilize periodically. Once established, these plants will require minimal supplemental irrigation or fertization. And it is best to site these at the outside of the dripline so that some rainfall and light will reach them, unless the tree has been limbed up somewhat.

The allelopathic properties of Thuja or even true cedars (Cedrus species) are overstated. They do tend to generate hormones and natural oils which make them less susceptible to insects as well as possessing natural fungal deterring properties, but the only thing that keeps plants from growing in close proximity is heavy competition from their aggressive and widespreading root systems for soil moisture and nutrients as well as the heavy shade they cast directly under their canopies.

thanks gardengal! maybe I will just slip some containers underneath with shade plants rather than going toe-to-toe with a big old greedy cedar. They look so bare undereneath, and take up a fair bit of our "greenspace" in our yard.