Weddingbee boards

I am not familiar with the logistics of something like this.  BUt I would think the best thing would be to somehow indicate a preference towards gift cards.  Maybe you can have a garden themed shower?  And on those invitations it can indicate local nurseries you like.  If you go for registering for trees and shrubs and perennials there are a couple of potential problems (from my gardening experience). 

1.  Assuming guests won't actually be bring a tree or plant to your shower/reception, the cost to deliver something would be a deterrent.  I don't know if they would even deliver perennial plants, so guests would have to deliver it themselves.  (Just kind of a big pain).

2.  If a guest is picking out the plant, you might not get what you want.  They might be unknoweledgable and pick one that is unhealthy, too small etc. 

3.  If you try to set something up with catalog/online nursery that ships, be careful.  Many many are not considered reputable.  OR can be hit or miss at best.

I think it's great you want to garden.  I love it.  As a guest I would get on board with a garden theme for your shower.  So you could register for tools, gloves, gardening books, pots and planters, Miracle Grow, fountains, bird baths, gazing balls, bird houses.  Knock yourself out.  But I think outside of buying those sorts of things for you, I would probably make it easier on myself as a guest and you too and just get a gift card.