Combine the potting mix with a balanced 10-10-10, 14-14-14 or similar blend fertilizer. Use 1/2 tablespoon of fertilizer per gallon of mix. A soil-less potting mix is preferable for strawberry pots, as mixes containing soil tend to compact and provide poor water infiltration near the bottom of the pot.
Drill 1/4-inch holes down each side of a 1-inch diameter PVC pipe, spacing the holes about 1 inch apart. Use a pipe 6 inches taller than the strawberry pot and don't drill any holes in the top 6 inches of the pipe.
Place the pipe inside the strawberry pot, centering it. Add the prepared soil mixture to the pot until you reach the first set of planting holes.
Plant a spinach seedling in each side planting hole, firming the soil lightly around the plant roots. Continue adding soil and planting the seedlings in the side until you reach the top of the pot. Plant two to three seedlings in the top of the pot.
Water each seedling by sprinkling water directly into the side planting hole. Pour water into the top of the PVC pipe to further irrigate the soil evenly throughout the pot.
Provide the spinach with full sunlight during the cooler spring season and some afternoon shade as summer temperatures begin to rise. Water the soil every one to two days using the irrigation pipe. Also feel the soil in the side plantings and water these areas if the soil feels dry.