The specific requirements vary, but plants generally need water, nutrients and sunlight to grow. They also need air, appropriate temperatures, space and time to thrive.
The main process that allows plants to grow is photosynthesis. During this process, chlorophyll in the plant captures energy from the sun. The energy is used to convert carbon dioxide and water into oxygen and carbohydrates. Plants use their roots to take in water from the soil.
Plants also require nutrients from soil. These include nitrogen, phosphorous and potassium. Plants use potassium to fight off disease and nitrogen for creating chlorophyll, while phosphorous makes flowers grow. Gardeners often add fertilizer that contain these ingredients. The optimal ratio for them depends on the plants.
Air is also necessary for plant growth, as they need carbon dioxide. They also need the temperature to remain in a range that is comfortable. Though the specific requirements vary by plant, many varieties thrive in warm days and cool nights.
Additionally, plants need adequate space and time. Since water and nutrients are drawn in through their roots, they need enough space for their roots to branch out. Overcrowding can not only strain available resources but also lead to spreading of diseases from plant to plant. Finally, they need time, which could be anywhere from a few weeks to a few years, to reach maturity.