Starting your seeds in egg cartons is easy, but some people find that their cartons dry out quickly. Rather than baby the cartons of starts, put them in a tray full of water, and let the carton wick the water up. Don’t overfill the tray – the carton should not be submerged as this will reduce oxygen flow to the young roots and encourage mold. However, keeping the bottoms of the cartons moist will ensure that the entire soil within is moist.
Temperature is important to starting seeds. Every seed has an ideal germination temperature. Checking with your seed supplier will inform you of the ideal temperatures for your crops.
While some folks will use heat mats or other electric devices, it is also possible to heat your plants using the sun. Within your cold frame or greenhouse, construct a Styrofoam box (or other insulated box) and cover the top with clear plastic to let in the light. This greenhouse within a greenhouse will keep your starts warmer than if they were simply on a shelf! Make the insulated box deep enough to put several water bottles on the bottom for the tray to rest on: the water will absorb the heat in the daytime and
A cardboard box works great. Use bubble wrap for insulation, stapled onto the sides. Aluminum foil on the inside of the bubble wrap and some black ink or paint on the outside of the box will keep it toasty. If you don’t have fancy polycarbonate sheeting for the top of the box, plastic food wrap works great.
Angle the top of the box so that the side facing closest to the sun is lower. This will prevent the box from shading the plants on that side of the box. Having a taller back wall will reflect the heat and light back down onto the plants.
If your water bottles are not warming enough, try adding quantities of salt to the water: as much as is possible! This increases the thermal mass of the water. Even if it sludgy, this is a good heat sink, better than bricks. You might also try using a bigger box so that sunlight will penetrate down beneath the starts and hit the water bottles.
In the coldest winter, stack or layer water bottles around the outside of the box. Be careful not to block sunlight coming in. You may also try other insulation, such as leaves or straw. If it is convenient, cover up the top of the start box with an old blanket at night.
Fertilizing: your plants are not in the soil and will quickly consume the nutrients in the little egg carton. Like all house plants, they will require fertilizer. Once your seedlings develop their second set of true leaves, it is time to start feeding them. Young seedlings are very tender and can't tolerate a full dose of fertilizer. Baby them with a half-strength dose until they are three or four weeks old. After that, you should start full-strength fertilizing every week or two. Composted manure, or a mixture of urine and ashes are good natural fertilizers. If you notice discoloration of the leaves – yellow or purple are typical – you likely have greater need for fertilizer.
If you notice mold, fungus or other microflora, you are watering too much. Reduce soil moisture quickly, and apply ashes, cayenne powder, black pepper, juniper juices, or other antifungals, or even transplant the starts into new soil.