I have an antique iron urn that is sitting outside and has begun to rust. Is there any danger to plants if I use it for a container/planter? Does it matter if I use annuals or perennials? I am also considering planting a yucca in it; how do you think it would do?

Your container will work well with no adverse reactions to rust by your plants. However you must drill several large holes in the bottom of the container to allow for drainage. To stop the soil from plugging the holes and restricting drainage, put a piece of screen or weed cloth over the holes. You may have a problem with the existing rust on the bottom—if you place your container on concrete or wood, the rust and water that drains out may stain the surface below. Annuals are always a good choice for planters; they do well in the summer and are not a concern when it comes time to put things away for the winter. Perennials (or the yucca) will require protection from the winter elements (such as putting the container in the garage). The same would be true for an evergreen or shrub. Perennials will also require repotting and division due to the restricted environment in the metal urn.