After some experimentation I have devised a method for getting the earliest green beans from your home garden. If you are willing to start and repot a dozen green bean plants (or purple green beans, sequoia is my favorite) you can have a nice sized harvest early in June rather than waiting till late July. The first thing to do is fill a dozen small dixie cups (4 oz. works great) with a mixture of seed starting soil and potting mix. The seed starting soil keeps the seedlings moist as they develop and the potting soil provides the nutrients to feed and nourish the young plants.
Most people start their seeds in the seed starter and leave them in too long not knowing that seed starter has no nutrients and creates weak spindly plants if they are not fed shortly after they sprout. So a mixture allows the plants to get a strong start and spend the first 10 days or so in the 4 ounce cups before needing transplanted. You will need to keep the soil warm at all times or the beans will not germinate. The best way is to keep them in the warmest room in the house near a heat vent or even better on top of the refrigerator which stays warm all the time. Always keep some plastic wrap on top of your tray to keep the moisture in but only until the plants emerge from the soil. Just be sure to check them every day because once they emerge from the soil they will need sunlight through a window. For best results move them from window to window throughout the day wherever the sun is shining. I put mine in the east window before I go to bed at night and move them to the south window around 11 a.m., then move them to the west window about 4 p.m. of course this will vary according to your location.
The more direct sunlight they get the stronger the plants will be. The beans will grow quickly once up and will need to be repotted in 16 ounce dixie cups when they are about 5 inches tall. Use some good potting mix (many advertise that they feed your plants for 3 to 9 months and these work best) and poke holes in the bottom of the cups so you can water from the bottom. The best way to do this is put all your cups in a tray and pour about an inch or two of water in the tray every few days then let the soil in the cups soak up the water through the holes in the bottom. This keeps the soil on the surface against the stem dry and encourages the roots to grow and seek the water at the bottom of the cup.
Many people make the mistake of thinking the soil must be wet on top for the plants to survive but this actually weakens the plants and promotes mold growth around the stem. So keep the surface dry and the bottom damp and your plants will be ready to set out in the garden as soon as the soil warms and others are just planting their seeds outdoors. Just remember that your plants can get sunburned just like people do so expose them to outdoor sunlight gradually for a few days before planting them in your garden. The best way to do this is to put them under a tree for a few hours each day where they will get filtered sunlight and after about 3 days of this they will be able to handle full sun in your garden.
If you fail to do this and put them directly from the windowsill into your garden the leaves will turn bleach white and the plants will die. Following these simple instructions will allow you to be harvesting your beans about 3 weeks after transplantation to your garden and about a full month before anyone else. Good luck and happy planting!