In order to weatherproof your artificial Christmas tree and keep the artificial snow from washing off in the weather while in your garden, you need to purchase a spray sealer. You can find this at any hardware store or large chain store like Wal-Mart.
This sealer can be clear spray paint or a clear sealent. The clear spray paint is the cheaper way to go, thus saving you money in the long run. Just be sure to spray on the snow first and then spray the clear sealer on top after the snow dries some. This will also give your tree a glittering look!