Oops, forgot to add that you need to figure out why you have the spider invasion in the first place.
Use the extension website to identify your particular type of spider and to find out what makes them feel at home on your plants-spiders are usually attracted by water and the presence of small insects they feed on, although there are some spiders that eat your plants, and others that use your plants to raise a family.
MooseMom is absolutely right about spiders being beneficial (mostly), so try hosing from underneath to dislodge them-stick the hose nozzle aiming upwards from the base of the plant and fire away.
If you decide you need something stronger, the dish soap is a great idea, but needs to be a mild one like Ivory (do they still make that?), and is a wonderful pesticide that is safe for the environment; you can also buy a product like Safers in the garden department or plant nursery. Check your local extension office website for good tips on spider infestations on plants.
Try a nice shot of water from the hose first, then do the dish soap (one squirt) in a 1-quart squirt bottle and spray directly on the spiders. Spiders are (mostly) good bugs, so you really don't want to kill them, just relocate them.
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