Purchasing new seed trays each spring can get really expensive. This year I am trying to supplement some of my trays with egg cartons. Throughout the winter I set aside our empty cartons (non-Styrofoam) to experiment with this spring.
First I ripped the top of the carton off, leaving just the bottom. We filled in each egg slot with potting soil and then added cucumber seeds. My assistant/daughter made plant markers for the carton so that we wouldn’t forget what we had put in there. After watering it slightly, it was ready to go.
Once it warms up enough you can either take the soil and seedling out to plant or cut the carton and plant as is. I hope this works out because it was certainly easy enough (and cheaper) to save our egg cartons.
Recreational Gardener 
I’ve never tried this but have heard it works well. You should know in a couple weeks.
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