I've used this for aphids (not on weed) but my only experience with spider mites my plants were way too large to 'dunk', and water alone did nothing when I cut branches off and submerged them. SO - if your infestation is advanced, and they have their little web palaces built, water alone is not enough imo.
Sulfur is a miticide.... and a preventative for pm, look for wettable sulfur it can be foliar sprayed. Always elevate plant to spray leaf undersides first, then reposition and spray from top down. Sulfur is thought to impart a bad taste cuz duh it smells kinda like sulfur - so bud washing is your friend.