If you have grubs in your planter boxes, they are likely there for only one reason -- to dine on your plant roots. They might have come in via new compost to your planter box or even been in the potting soil or container packaging of a new plant. Or, if you see beetles around your house, they might have been laid in the soil by the adult insects. Because a planter box is a small, finite space, it is better to replace contaminated soil rather than attack the problem with chemicals or introduce nematodes or bacteria into the soil.
Turn over the top 2 to 3 inches of soil with a hand cultivator in late summer or fall when grubs are closest to the soil surface. Hand-pick the exposed grubs from the soil and continue to turn the soil until you no longer find any grubs. Check the soil the next day to see whether any new grubs have moved near the soil surface, especially if you have a deep planter box. If the infestation is severe, or you want to be sure you get all the grubs, replace the soil.
Carefully dig around the root zone of each plant in the box, using a trowel. Try not to sever the roots. Scrape some of the soil from around the base of the plant to better see the roots.
Gently lift the plant from the planter box by the root ball and set it aside. Continue removing the plants until there are none left in the planter box.
Dump the soil from the planter box into a plastic bag or other container for disposal so you don't transfer the grubs from your planter box to your yard or compost pile. If there are only a few grubs remaining, you can sift through the soil and remove the grubs to compost the dirt.
Add fresh potting soil to the planter box. Fill the box with enough soil that the plants sit at the same level they sat at before you removed them from the planter.
Lift each plant by the root ball and check the roots for any grubs, remove any remaining grubs, then place the plant back into the box. Repeat for all remaining plants.
Fill in around the plants' roots with potting soil until the roots are covered to the same depth they were previously. Water the soil to settle it and add more soil if necessary.