Tell the preschoolers that you are going to learn about plants and gardens, and ask them if they have any plants and gardens at home. Along with displaying real plants, show the preschoolers pictures and photographs of a variety of other plants that cannot be brought into the classroom. Also show them photographs of gardens. Monet's garden in Giverny is a good choice and can be found online. The link is given below.
Learn about plants
Talk about what plants are. Living things made of the following parts: roots, stems, branches, leaves, flowers, and fruit.
Learn about seasons and environments
Talk about the different seasons---summer, winter, spring and autumn---and different environments---deserts, forests, grasslands, mountains, lakes, ocean. Talk about how plants and gardens are affected by seasons and environments. Show the preschoolers photographs of plants in different settings. For example, plants in a desert, plants in a tropical forest, a garden in the autumn, a garden in the spring.
Learn about gardening
Talk about gardening. Ask them what a garden has? Plants and trees and shrubs and creepers and flowers, of course. How are these planted? Show them a chart detailing plant growth. What are the different implements used in a garden? For instance, a hoe, a spade, and a rake.
Learn about planting a garden
Take the preschoolers to a garden patch and show them how to dig holes and plant seedlings. Then water the seedlings. Tell the preschoolers that plants need soil, water and sunlight to grow. If they don't get these, they won't grow or grow big enough. Plants also need the right environment. Get quick growing plants like pumpkins and have the preschoolers observe how these grows.
Learn about the uses of plants and gardens
Plants and gardens create a pleasant atmosphere and make us happy. Some plants are edible---you can eat their stems, leaves, flowers or fruit---and are planted in kitchen or vegetable gardens. Some edible plants are asparagus, lady's finger, tomatoes, and cauliflower.