If you're setting up a garden and have covered some areas with black plastic mulch, you know you have to provide some sort of irrigation system for the plants in the mulch. However, you have to keep rainfall in mind as well. Black plastic mulch is basically just a sheet with no openings other than the ones the plants poke through. Any rain that falls doesn't really reach the plants unless the rain falls into or runs into one of the openings. In heavy rains, that is a lot of water running off the plastic. You must provide a drainage channel, or you risk washing away soil that's next to the garden bed.

Soil Saturation

A lot of rainstorms aren't going to be heavy enough to wash away soil -- the rain will just soak into the soil. But if you get storms that are strong enough to saturate the soil so that it can't absorb any more, the area around the mulch will be in trouble. Not only will it have to deal with the rain that's hitting it directly, but also the rain that's running off the mulch. Some of the extra water would spread to neighboring soil, but eventually, the soil would begin to wash away.

Drowning Plants

Another problem with too much water running off the mulch is that it could drown the roots of nearby plants that need well-draining soil. After a point, heavy rain will cause water to accumulate faster than it can drain away. Any roots in that area will receive too much water, potentially harming the plant.

Drainage Route

Give the extra water a way to safely drain away from the bed without forming puddles in saturated soil. Create slopes that lead the water to storm drain systems, or down toward plants that prefer extra-moist soil. Or, build a French drain that allows water to soak down into a perforated pipe that lets the water drain out of the yard.

After the Rain

Once the rain has stopped, inspect the mulch and the soil around it to see how the area fared. Adjust drainage systems accordingly if you think there are areas that are still getting too much moisture.

If you need more advice about mulch and rain, talk to garden centers and landscaping companies about the mulches they offer. They can give you specific information about how the mulch performs when rain is a factor.