When you mail in your application for naturalization, you will have to include a copy of your valid green card. If your green card is about to expire and you want to apply for U.S. citizenship, you should renew it as quickly as possible.
When your green card expires, you are technically still a lawful permanent resident. Your status as a permanent resident never expires, but the green card acts as proof of permanent residence. You usually must renew your green card every 10 years.
Technically, you still qualify to naturalize even if your green card is expired. But when you apply for citizenship, your green card should be valid for the next 6 months after submitting your naturalization application. This is because the naturalization process typically takes around 6 months to complete. So, if your card is about to expire, you should still renew it.
Keep in mind, you will then need to pay both the application fees for renewing your green card and applying for naturalization separately. This can get expensive, so it's best to plan ahead.
Your green card is proof that you are a lawful permanent resident, and you will need it to prove your immigration status even if you have a naturalization application pending.
Not renewing your green card can impact your status in other ways too. If you travel outside of the United States, you must present a valid green card in order to reenter. Your green card is also proof that you have work authorization in the United States, and is necessary to receive any sort of government benefits. Renew your green card as soon as possible to avoid any problems, even if you plan to apply for citizenship or already have a citizenship application pending.
No matter what, you should renew your green card in case USCIS delays or denies your application. If you have more questions about whether you qualify for naturalization, consult an immigration lawyer who has experience helping clients with US citizenship.