Some chemicals used with power washers can kill your plants, but there are safe chemicals you can use when power washing. Chlorine bleach will harm plants, while oxygen bleach will not.
If you need to power wash to address a moss problem, you can find products that target just moss and keep plants safe. You can find effective power wash chemicals that will not harm plants.
Check the label and look for warnings to determine if a product is safe or will kill plants.
Contracting with Companies
If you hire someone to power wash your home, ask him about the chemicals he uses. Communicate with him that you don't want any plants killed during the process.
There are many power wash companies that use environmentally safe chemicals. A reputable company will also take steps to protect your plants from the chemicals if there's a chance of harm.
Don't assume any company will use the right products or protect your plants; ask to be sure.
You have two options to protect plants during power washing: covering them or rinsing them. Securing plastic tarps over all landscape plants prevents the chemicals from touching them to keep them safe.
If you plan to do this, make sure the tarps have no holes. On hot days, tarping could smother the plants by cutting off access to air.
In hot weather, pre-wash the plants before you start using water. Rinse the plants off during the cleaning process and afterward to prevent the power wash water from drying on the plants.
How often you'll need to rinse depends on the heat.
If you're tempted to skip the chemicals because you're afraid they're going to harm your plants, don't. Chemicals help break apart the dirt, moss, mildew or other debris that's clogging your roof or deck.
While you could get most of it off with high enough pressure, this will seriously damage the wood.