Here's What You Can Substitute For Lemongrass
Lemongrass is a tropical plant known for its tangy citrus flavor, and is an ingredient that is generally used in specialty dishes such as Southeast Asian cuisine. As a result, you can easily find yourself in the middle of making a dish and realize that you don't have any lemongrass on hand. Due to its unique herbal-lemon flavor, it's difficult to replicate, but thankfully, it's not impossible.
When you need a substitute for lemongrass, Spiceography has a solution — they suggest using lemon zest. Lemons are easy to find, and to get zest, you can just run the lemon along a grater, bringing the lemon flavor into your dish. When using this method, one lemon is equivalent to two stalks of lemongrass. This method won't replicate the herbal flavor, so to get that, add some arugula. A good proportion to use is one teaspoon of lemon zest combined with a single arugula leaf to substitute for the flavor of one stalk of lemongrass.
Other lemongrass alternatives
Another possible substitute is to use coriander (also known as cilantro) and fresh ginger (via Tastessence). Keep in mind that ginger is a strong flavor, so you don't need a lot. The best way to go for this method is to grate the ginger, and then, along with the ginger, cut up coriander stalks (not the leaves!) into small pieces. Use two teaspoons of ginger with two teaspoons of coriander stalks to replicate the flavor of one stalk of lemongrass. If you have to use dried ginger, remember that the flavor of dried ginger is concentrated so you'll want to use less. McCormick suggests subbing 1/4 teaspoon of ground or dried ginger for a teaspoon of fresh, so for two teaspoons of fresh ginger, use 1/2 teaspoon of dried ginger.
If you don't have lemongrass, there is a possibility you don't have ginger, coriander, or arugula. If that is the case, don't fret — you still have options. Lemon juice in small amounts can be used as a substitute for lemongrass (via Greedy Gourmet), and in a pinch, you can even use limes as a substitute. As with the lemons, use the lime zest. However, lime zest is best in recipes that only require a hint of lemongrass. For those recipes requiring more, it's better to stay with lemons.
If you use lemongrass regularly, keep some dried lemongrass on hand. Then, if you find yourself out of fresh, you have the option of using the dried, which is going to be a closer flavor to fresh lemongrass than other substitutes (via RawSpiceBar).