A spider mite infestation on potted plants is far from a welcome sight. The pests do indeed look like spiders under a microscope, but to the naked eye you're more likely to see them as orange dots on the underside of leaves. At their worst, spider mites can defoliate plants or cover them in webbing. Avoid these threats by submerging potted plants in soapy water. This method removes and kills the pests.
Because the most effective way to submerge plants requires you to hold the inverted pots, it's not comfortable to hold them for longer than 30 to 60 seconds. Fortunately, that's all the time you need for the fast-acting method. Spider mites cluster on the foliage and other above-ground plants and are easily loosened in water.
If your plant is heavily infested with spider mites, you may need to repeatedly dunk the plants until you no longer see signs of the pests. If you see more than a little stippling on your plants, or if the undersides of foliage is covered with orange spots, reserve enough time to dump out the used soapy water, prepare a new water bath and swish the plants at least one more time for up to 60 seconds.
Soapy water is the best medium for submerging plants. While large tubs are obviously impossible for a home gardener to submerge, any pot that you can hold comfortably in your hands for about 60 seconds can be held under water. Choose a container or sink at least as deep as the plant's height, and which has the circumference to allow you to move the pot around. For every gallon of water you use, add 1 tablespoon of mild dish-washing soap or other mild liquid soap. If the exposed part of soil on the surface of the pot is wider than the span of your hand, tape plastic wrap onto the pot so that it covers the soil.
The last thing you want when submerging your potted plant is for the plant to slide out of the pot into the water. To prevent this, place one hand over the soil's surface and press down gently. Avoid crushing the stem between your fingers. Turn the pot over with your other hand and lower it until the water is up to the soil line. Move the inverted, submerged plant through the water in a swishing motion for about 30 to 60 seconds. Turn the plants upright and allow them to dry. If you've taped plastic to the pot, remove it.
Submerging plants is an effective way to treat new plants from the nursery or newly-infested plants. But if you are dealing with a large number of plants, or with those growing in large containers, the submersion method isn't practical. Instead, try blasting the mites from foliage with a garden hose. Putting soapy solution into a spray bottle is also effective, provided you coat both the tops and bottoms of foliage when spraying. Use the same ratio of soap to water as the submersion method. In addition, be sure to eliminate the dry and dusty conditions that encourage spider mites. Keep plants well-watered, and run a hose over un-mulched paths and gardens.