My green scallions go dehydrated and become no longer fresh after they are placed in the fridge for a few days. The white part usually stays good for longer, but the green leaves go dry easily. Is there any good way to keep scallions, especially the green leafy part, fresh, for a longer time?
Scallions are very touchy. You don't want them to dry out, and yet you don't want them too wet or they will get slimy and disgusting.
What works best for me is remove any binding holding them together (e.g. rubber band or twisty-tie), (and certainly trim off any parts of the scallions that are already damaged and/or slimy), don't wash until you are ready to use them, wrap the bunch of scallions loosely (but completely covered) in a paper towel, and then store that wrapped bundle in a flimsy plastic bag (maybe the one you brought them home from the supermarket in) in your refrigerator.
Sounds "fussy" when you lay out all the steps like that, but really, it's not a whole lot of work. Removing the rubber band etc. prevents damage where it would cut into the veg. flesh, paper towel absorbs condensation & liquid water which will rot the scallions (paper towel will prob. become slightly damp after a while in the fridge, but this is OK), plastic bag prevents evaporation of moist air.
I find this works well for lots of greens, esp. cilantro - the worst offender in the demanding vegetables category.
P.S. If you aren't too squeamish, you can even remove the very outermost layer of a scallion which has begun to "slime-out", wash the remainder right before using - & it's fine!
Tender leafy vegetables preserve best when they are cold, at high humidity, but are not wet themselves. The refrigerator has the proper temperature, but you have to do something about it drying them out.
What I find works better than the paper-towel-in-a-bag method is the over-water method. In this, you take a tightly closing container and a trivet that fits into it (there are also specialized plastic contraptions for this). Put about 0.5 to 1 cm of water on the bottom, place the trivet on the water and pile your trimmed vegetables loosely on the trivet. Close the container (it should be roomy enough that the lid does not compress the vegetables) and put it into the fridge. That way, you get about 5 days of freshness.