Alright, so it was already established that drainage holes are a key design feature to keep an eye on. Yet, even with plants that are not meant to rest in standing water for extended periods of time, you can use a garden urn without draining holes for planting and here is how. One method is to merely use a little bit of water at a time. Given the fact that every drop of water that goes inside will be stuck there, you need to disperse it slowly and sparingly as it will accumulate at the bottom. But, when it begins to pour down rain, this theory is out the window if you forget about it.
The more logical method is to add a drainage layer. Before adding the soil, go ahead and place a layer of either pebbles or stones on the bottom. With this added, it essentially becomes a barrier between the water and your plant.