Normally onion plants could handle 31 degrees just fine, so no, I wouldn't cover them up generally at 31 degrees. However, only you know how warm it has been lately and how much exposure your onions have had to cold weather in recent weeks. If your onions have had a lot of prior cold conditioning, they should be fine. There are exceptions though: 1) If it has rained a lot and they are standing in very wet soil, they are more likely to suffer damage, so I'd cover them up to hold in whatever heat the soil and the plants have accumulated today. 2) If your plants have had a lot of prior conditioning to cold, I wouldn't be worried about them unless all they have had the last week or two is warm weather. Often, once we start warming up, the plants sometimes can lose some of their prior cold conditioning. Certainly, mulching them or throwing a sheet or row cover over them won't hurt them at this point. Good luck with them.

Dawn

I'm not worried about most of my plants, however, I have radishes and arugula that have just sprouted so I'll throw some leaves over them.

No . ,I'm not worrying about my onions.They should be able to handle it OK.

If it don't get below 20 some degrees it should be OK.