I think with sunglasses, it’s less about high price than how they’re constucted. You want something that sits tight to the face (without being so tight it hurts), has fully opaque frames, and polarized lenses. My actual glasses darken for sunlight, but they don’t sit close to my face, so on really sunny days I switch to contact lenses plus sunglasses.
Take breaks? But…I’m gardening! 
I don’t sweat much, so I think I am really underestimating how much water I need to drink while working.
So, bigger hat, better sunglasses, more water, more breaks - I can do that. Thanks, everyone.
While I suspect it’s not what she’s going for, Paris Hilton’s outfit here is probably a good gardening choice. 
I’m gonna jump on board with the “water, water and more water” folks. Drinking lots of water BEFORE the onset of the headache should prevent it altogether. The first time I went to Key West, I barely drank any and got a terrible migraine afterward. The next time I went, I made sure to drink water while we were out walking, and by the end of the day I was fine.
Polarized sunglasses. Ones with very wide temples (to block extra light). The bigger the better.
Sunglasses that say UVA/UVB, don’t bother. It’s just a coating, and nearly all sunglasses (and regular glasses) have it anyway. If you’re prone to outdoor headaches, try out the polarized - the lenses have a filter that cuts out glare. Makes things much easier on the eyes. Less eyestrain, less headache.
Grey, dark grey, black polarized lenses - good for driving and general use (doesn’t distort color vision)
Brown and amber polarized lenses - good for golf and other outdoor sports (improves depth perception a bit)
Don’t use the sunglasses when you are working outside.
You may very well have a sinus headache from allergies triggered by working outdoors. No amount of OTC pain killer is going to help a sinus headache because the passages have swelled shut and are filling with fluid. It is necessary to get the passages to drain to relieve the pain.
It took many years for me to recognize a sinus headache. What I discovered is that they trigger tension headaches in the back of my neck. If this goes unchecked I get a migraine headache. The result is a very painful headache that will not go away for many hours.
At the first sign of such a headache I take sudafed and ibuprofen with a caffeine drink and I also use nose spray depending on the severity. Note: all these things raise your blood pressure.