Young seedlings need 14-16 hours of light per day for optimal growth. Choose a south facing window, with no trees or buildings to block the sunlight. Supplement with full spectrum lights designed specifically for growing plants or use easily found fluorescent lights, combining one cool white bulb and one warm white bulb. Insufficient light will lead to stretched out « leggy » plants. A healthy tomato plant, for instance, should be almost as wide as it is tall.
All seeds are different, but many need temperatures of up to 25C to germinate. Once the seeds have sprouted, keep daytime temperatures between 18-22C and nighttime temperatures between 15-18C.
Water in the morning and keep soil moist, but not waterlogged. Too much water may result in stunted growth or a fungal disease called damping off. Use a spray bottle to mist your seedlings twice a day to keep relative humidity high.
Pay close attention for tell tale signs that things aren’t quite right. Inspect plants for damage and look on the underside of leaves to see if insects are present. Look for pale, yellow or purplish leaves, indicating a nutrient deficiency.