What do my fellow backpackers use for sleep cover during the summertime?  I have a 50 degree quilt and find that to be too warm in the southern states.  I am thinking I’ll just get a Sea to Summit silk sleeping bag liner.  Any other lightweight ideas?

I sleep on top of the bag in the summer not in it.

I have a synthetic 30ºF down bag that I just unzip and toss over me like a quilt.  If it's too warm for that, I just push it to the side and drape a jacket across my chest, or sometimes nothing at all.

Subtle differences in sleeping pad material can make a sizable difference in warm weather - my X-Therm pads are, ironically, more comfortable in warmer weather than my Uberlite pad.  The Uberlite has a smoother fabric than the X-Therm, and thus, it can feel a bit clammier at times.  Some fabric between you and the pad will help, but it needs to be readily washable: body oils, trail dirt, you name it, it's going to get ground into that bottom layer like a geological process.

I've been in the ~40ºs in June at higher elevations on the AT.

As you said, the silk liner can be a good choice.

I sewed my own synthetic, taffeta-type liner decades ago, and it has worked well in warmer temperatures.  Today, store-bought liners are easy enough, though, I'd be tempted to pull open some of the seams, and re-sew them with some fasteners just to allow easier in/out.

Sea to Summit and Klymit both make a hot weather sleeping bag liner that can be used with or without a sleeping bag. They are expensive so I've always just laid on top of my Therm A Rest and used a ponch liner. A small fan in the tent makes a really big difference. I take a bunch of rechargable batteries and rotate them out.

3 GI poncho liners, fold one as a ground cover/ mattress, one as a pillow, and one to cover you. Plenty comfy, can be folded pretty small in a compression bag in seconds, and if rained on they dry out quickly.
Retired 11B

Snugpack junglepack.

I mostly use a hammock these days too.

Since asking this question, I’ve been on two trips and found one system that sucks, and another that’s the cats pajamas.

Sucky - Tennessee - Iron Mountain Gap to Highway 19 - Roan Highlands on the AT in June.  Highs in the high seventies, lows in the high fifties.  Tried using a sleeping bag liner on an insulated big Agnes pad and slept cold.  It was crap!

Cats pajamas - Georgia - Amicalola Falls to Tesna tee Gap in July.  Highs in mid eighties, lows in the mid sixties.  Enlightened Equipment Revelation fifty degree quilt on a Thermarest NeoAir XLite pad.  Unzipped the foot area and kept my feet out overnight.  It was perfect!

I have a couple of dry-down quilts. One is a custom 50-degree quilt and is literally the size of an issued poncho liner but compresses like crazy. Second is a Sierra Stealth down quilt from JRB rate to 40 degrees.

We use hammocks, so it's surprising how cool you can get when only 3000-4000 feet of elevation, temps around 70 degrees with a nice breeze after a thunder storm or three.

The quilts just work better to help thermoregulate; especially to cool off and not sweat. I usually keep my core and arms covered and let me legs out to help radiate unneeded heat.

ROCK6

My secret was ordering a custom size quilt just for summer use. It was an extra expense but the idea has panned out for me, here in GA.

Although it's temp rating is 40F, it is only 50" wide (5" taper) and 60" long. I am 6'2" and 235 lbs.

I start out sleeping with it stored down by my feet. Around 4am or so, when I start getting chilled, I pull it up over me. It covers enough of me that I am warmed up, but still am well ventilated due to less than full coverage.

I can also turn it diagonally for more lengthy coverage and can surprisingly get a lot of warmth.

I ordered from this company Loco Libre

I end up camping in July in SE Pennsylvania, usually a week in the summer at scout camp. I have an Exped Synmat pad and a Cocoon coolmax travel sheet. I'm usually good in that, but I bring a cheap fleece zip-up sleeping bag for that one night when temperatures become pleasantly cool. On the nasty nights, I have a rechargeable fan.

My Jungle Bag has been one of my best purchases. I've slept with it over me from 40*-90*. Keeps the bugs off, too.