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Lemongrass is a plant that looks like grass, smells minty and tastes similar to lemons. However, lemons are not a good substitute for lemongrass.
Keep fresh lemongrass in a jar of water, it will stay fresher than in a fridge. Lemongrass's stem and leaves are used. For soup, my mother would fold the whole lemongrass a few times and tie it leaves around the fold. Then she would drop it in a pot of water.
Lemongrass tea - my mother's recipe. Slice up lemongrass across the stem. Sun dry or oven dry. Toast it in a pan until light brown. Steep a teaspoon in hot water. It is ready. Or if you like it cold, add sugar and ice.
If you live in Southern California or other warm climate, you can plant the lemongrass outdoors. Get a few stems from an Asian grocery store or farmers market, put them in a glass of water. You need just an inch of water only. Let them grow roots before sticking them in the ground outside. Cats love them.