Sensitive plant in egg carton planter
You can use any plastic egg carton to make this planter. The best would be one with a clear top and individual egg holding spaces on the bottom.
Plastic egg carton
After finding your carton, gather a sterile, light weight, fine particle soil mix to improve your chances of success. I use a pre-mixed seed starting mix to make things easy. You can also mix your own with a mixture of peat, perlite and/or sand. Soil from outside would work, but consider the following:
Natural soil may have rocks and sticks that can damage roots or prevent emergence
Natural fungus, bacteria, and other organisms may be present that may damage your seed or plant
The soil may not have the ideal mix of drainage and water holding capacity, which may increase problems with fungus and bacteria
Weed seeds may germinate and it could be hard to tell the difference between them and what you want to grow
Seedling mix before (top) and after (below) adding water
The mix needs to be moist before planting the seeds. If we add water after planting, it may cause a little disruption to where we placed the seeds. Add some water to your mix then use your hands to squeeze the mix and water; this should allow the mix to soak up the water. If you have the correct amount of water, you should be able to form a ball with the mix and some water will be released when squeezed. You’ll want to squeeze moderately hard to remove water so the soil is moist but not sopping wet. Place the mix into the egg carton, lightly packing it so that it is below the edge of the rim.
Fill height for mix in carton
You can now take any seeds you want (vegetables, fruits, flowers, trees, or shrubs) and place 1-3 seeds into the soil as per the directions for those seeds. If you do not have directions, most seeds do fine when covered with enough soil such that they are not exposed when watered (depth should be about twice the size of the seed, some will be deeper if they are larger seeds).
Egg carton mini-greenhouse with soil and seed in sunny window
Close the cover and place in a warm location until you see green leaves or stems starts to poke through the soil. Depending on the plant, it can take anywhere from a day or two to several weeks. At this time you can move them to a sunny window or under a grow-light. Keep in mind that our eyes are very good at seeing in low light, so even if a room looks bright to you, it may not be bright enough for the plant to photosynthesize effectively. If the plant is growing very long and stretched, it probably needs more light.
Native Blue Flag Iris (Iris versicolor) sprouts in egg carton planter
You can leave the cover on until the leaves touch it, then you should be safe to remove it and water the mix. Water until you just start to see the water pool at the bottom of each cell. If you have another tray to place underneath the egg carton, you can poke a hole in the bottom of each cell before planting for water drainage. Once the plants get big, you can move them to another pot with normal potting soil, or take them directly outside if the weather is compatible with your plant type.