Does spending time in the garden decrease stress?

Yes, it absolutely would be a tranquil and relaxing setting.

Gardening can be relaxing if you like that and mom is able to help. Otherwise like others suggested start with a couple patio/deck pots. Fresh air, sunshine help to make one feel better. I sometimes get stress headaches and a drop or two of Lavender oil (make sure it's pure essential) at temples or back of neck really do help. Having a quiet place, whether indoors or out, where the individual can relax and not feel any demands of the day can do wonders.

Mojorox has some great advice. Small, manageable planters and pots might be the answer for you to make your mother happy. You don't have to invest in a major overhaul of your yard.

To add to my previous comment....I spoke about my experience with gardening but not my mom's. She absolutely loves flowers and trees and gardens. She even loves weeds if they flower. But she does not garden herself. I don't remember her ever gardening really.
I do up rail planters for her in early March on the front deck. She is in charge of those since they are easy access and no kneeling, crouching, bending, lifting etc. I figure it's the price she is capable of "paying" to be surrounded by the gardens she loves so much in the yard. I also set up a gazebo with swing on the back patio where she can sit in the shade and watch me slave....I think she likes that part too. Lol
We call it the backyard bistro and it's set up with an eating area, shelves with books & magazines, a puzzle/games table. When flowers are ready to cut, i prepare a bouquet and place it on the bistro table for a surprise the next mornimg. She loves it and It gives her the opportunity to come outside and be comfortable and enjoy what's going on away from the TV. Every so often, she gets her hands dirty with the plants too, so, bonus.
I do my best to work with and be satisfied with whatever the situation calls for and not push for more than she is capable of.
Sometimes a little thing rewards us both with more than either expected.
I make sure I stop and appreciate those moments so i can remember that feeling on other days when I'm feeling angry or negative.

Even if your mom liked gardening, I still wouldn't overhaul the landscaping. I would start with some sort of raised bed that's out there. Can be built or bought, but doesn't require kneeling/bending/stooping to maintain...Maybe can even wheels on it to move to shade if it's a particularly hot day when she goes out to work on it? If you enjoy cooking, maybe a small herb garden mixed in.

Aside from that, and not knowing any details, let's hope your mom doesn't have a more serious health issue because, with a doctor like this, she could be in trouble.

The cure for this ill
Is not to sit still
Or frowst with a book by the fire,
But to take a large hoe
And a shovel also
And dig 'til you gently perspire.

And then you will find
That the sun and the wind
And the djinn of the garden too
Have lifted the hump
The horrible hump
The hump that is black and blue.

From "How the camel got his hump," one of Rudyard Kipling's Just So Stories. Mind you, the particular 'hump' he's talking about (the whole poem goes on quite a bit) are the blues that descend from "having to little to do" - which may apply to mother but probably rather less so to the caregiver.

For some people gardening is a blessing, for others, an evil chore. Years ago I hated yard work of any kind. I have bad hayfever so even mowing the grass was avoided at all costs.
One day, after moving in with mom, I had the worst day of bad days at work. The stress of the job at that time was a killer. I was angry, on the verge of tears, and in no mood for anything. At home the lawns needed mowing badly and with rain coming next day, it had to be done now. I was swearing up a blue streak, kicking the lawnmower as I rolled it out to the front yard. Oh, I was hating life right then.....
Mowed front with enough time to also mow the back (not small yards either). That looked good enough to make other areas look bad...so out with the trimmers and clippers I went.....Awhile later, I noticed a miracle....I was no longer stressed or angry. Tension in my body was gone, my headache was gone, and I was humming without a thought of work in my head. I felt positive about things and felt excited as I watched the yard shape up. Had you told me this years before, I would have thought you stupid or crazy or both.
Flash forward 5 years....it's one of my favorite things to do. I love it. When I feel icky I go play in the yard. From easy clipping flowers to hauling giant rocks, making my own soil to chopping down trees, building trellises or redesigning the layout there's something for any mood I might be in. There is also something about working the earth with your hands that is very grounding. Watching something you've nurtured grow and thrive is peaceful. Seeing your accomplishments right in front of you is a huge boost.
Now that's just me. I think people either love it or hate it. But it doesn't have to be all or nothing. I think it's the creativity involved that matters. Take this idea and tweak it to suit your mom's interest (if she has any).
You could do a large planter scene. You pick a theme and build a village or fairy forest (or whatever you like) using miniatures or around the house items. It's a putter project as you build/change it/watch it grow. It's super creative without the chore gotta get this done feeling. Create an area for mom to work on it that works with her strength and mobility etc and there's no heavy lifting...yayy.
Any idea you have can be tweaked or altered to suit your needs. Gotta think outside the box. There's hundreds of neat ideas online too. Start small and build on things as your mom progresses on her ability. Another thing I do to destress is paint. Paint anything from walls to custom wine bottles. Again, I think it's the creativity of it.

I find there's never just one answer to a problem either. If the doctor was right and just garden to alleviate the problem we'd all be landscapers now, wouldn't we. I know your hands are full but you are her advocate. Research on line different things that could be a cause or solution. Try different things. Works? Great. Not work? Next.
Before any doctor app of mom's, I research what my concerns are, what I'm looking for, and ask opinion on what I'm thinking of trying. I put the doc to work. I don't just take their response as the golden word. If the doctor doesn't work with me, or is always shutting me down, I find one more suitable. Not to hear what I want, but one who interacts with us on all levels of health. I do believe in Mind/Body/Spirit and all three must be nurtured to have health and peace. I don't want a doctor that minds the body but ignores the rest.

Hope I helped even a tiny bit.
Good luck.

I like being outdoors if the temperature is perfect and I am dressed appropriately. I love flowers that someone else has grown. Our local botanical conservatory is a very stress-relieving place for me. I have lots of indoor plants. Up to this point outdoor gardening has just been a stressor for me. I might give it another try this summer.

Would gardening have relieved stress for my mother? OMG NO! But she loved her indoor plants.

Gardening as therapy? Yes.
Your mom enjoys it... Absolutely!
It doesn't require a total overhaul. Start small if necessary - purchase a small indoor plant, some potting soil, a nice ceramic pot and simply repot the plant. If your mom is up for more, consider some easy maintenance herbs which can be planted near the back door or along a walking path. Rosemary and mint release a nice fragrance when brushed or rubbed between fingers. Double bonus.

We have an arboretum close by that my mom likes to visit. She usually uses a walker but for somewhere like that I load up the wheelchair and we go over and I push her along the paved trails. During school days you can avoid alot of family groups. It's free and she enjoys even a short visit. Sometimes we just go over to a garden center and look around. She uses walker then.

I'd consider creating a small oasis area rather than a major backyard redo, unless the redo fits your overall home plans. There are also the planters that allow you to garden standing, sitting or if you're in a wheelchair. Plus with a smaller area to work, it would stay neat and be a joy, not a chore. Check Gardeners Supply for these. They also have large self watering pots.  And yes, for me, it's a real destresser to dig in the dirt.

I think your dr sounds a tad patronizing....like the dr who told my mother that we kids were the source of all her stress, etc and told her to kick the washing machine(?) needless to say, our washer always had huge dents in it. How weird is THAT?
As for the gardening--I personally LOVE it and seeing my yard in pristine condition and the flowers and veggies thriving--well, gardening CAN take one out of the day to day and relieve stress if you like it. If you don't it's just another chore.
The Vit D you get naturally from the sun is great--but I then slather on the sunscreen. I can happily waste a whole day mucking about our small yard.
If mom likes to garden, then go with the raised garden beds she can really see and won't have to kneel to work in. You can control the growth and many fewer weeds in those.
Don't hang outside in the heat of day--that would give anyone a h/a. Also, make sure you stay hydrated while working outside, you sweat and don't notice it and THAT can give you a headache in itself.

I'm fat and out of shape. And I cannot bear the heat, or strong sun. (I sound pathetic, huh?) I get headaches from doing yard work in the summer, the heat and the stress of getting back up! lol. For stress relief, I like to color in my mermaid coloring book, go for a walk in nature, read, cook...so it all depends on what you mother finds reducing stress for her entails. Frequent headaches have many causes.

Before you spend big bucks go to local gardens that welcome visitors - if mom gets benefit then start small - maybe mom was a gardener & can input ideas - if she wasn't then almost anything will do - if your yard is terrible then fixing it will increase property value - don't go overboard spending thousands where a few hundred will do the job

Many people swear to the benefits of "earthing": touching the ground with one's bare feet or hands, and touching plants rooted in the ground. Gardening, or being in a garden, has earthing benefits. I know I feel better when I garden or am in the garden. Even just a small patch of plants (no toxic herbicides or pesticides, please!) can work wonders.

Nature, even a tiny little patch of garden, away from traffic of all sorts (people included) and a comfortable bench (with back support) can be a heavenly secret place just to relax perhaps in the shade of a tree...add soothing music (with earphones if necessary) and when the temp isn't over 75 (even early morn), this can take the place of any therapy. There are lots of plants that require low maintenance. We have created Mom's slice of heaven on her deck and take care of the potted plants ourselves.

I remember back in the 1950's and 1950's, some doctor would tell a patient who comes in feeling not up to par, for women to go out and buy a new hat.
I wonder what they told their male patients?

Just being outdoors brings a physiological response: blood pressure and heart rate decrease, and deeper breaths are taken. The doctor's advice was not a patronizing pat on the head, it was good advice. Mom and I would "tour" the yard, looking at and remarking on flowers and plants as they budded and blossomed. It might be a good idea to spend some time with your mother in the garden (or anywhere else she would enjoy being outdoors) before sinking time and money into a major landscaping that might not be necessary, and might be stressful in and of itself. Hoping your mother finds relief from those headaches.

Gardening or any activity outside naturally reduces stress. It is always best to try natural "cures" rather than medicating every time there is an ache or pain or problem. If the headache seems severe, she should be thoroughly checked in case it is an aneurysm or dangerously high blood pressure.

I didn't see where you are from but where I live it can already be in the 70s at 7am and only get hotter. I'm in my 50's and garden work isn't a good time its more of a "got to". Indoor relaxation would probably be more my thing. You know your mom etc judge by that. As somone that wears glasses I know she'd need prescription sunglasses for sure.

It's been a long time since I've come across the kind of doctor who would pat someone patronizingly on the shoulder and tell them their symptoms are all in their head, I've got to say the idea gets my hackles up. There are lots of ways to relax and de-stress, gardens can be lovely, but so can the beach or meditation or a day at the spa, there is no one size fits all.
And given his less than helpful treatment plan I wonder how deeply the doctor bothered to look for a cause? At the very least excess anxiety can be treated, and so can pain.

It depends on the person but yes, gardening can relieve/reduce stress.

Some people love "playing" in the dirt. And sunshine gives Vitamin D.

Yes.

Yes, yes, yes.... gardening does help with stress big time, but only if one enjoys doing that type of work.

I remember back when I use to be out 8 hours on a weekend pulling weeds [great stress relief, just think of that weed as someone who has ruffled your feathers].... putting down heavy bags of mulch... trimming bushes... etc.

BUT, as we age we cannot enjoy gardening as much as we are not nimble any more. I found myself squatting down to pull weeds then I couldnt get back up... oops. So you will need to take that into consideration for your Mom if she is physically able to garden.

Or just go to Home Depot or Lowes, and pick out some flowering plants and put them on the deck, front porch railing, or where ever and let Mom be in charge of watering and pruning. Place the flowers where she can see them. There are large plastic light weight flower containers that are great for a half dozen plants.

My backyard was looking like a wild habitat so I hired a fellow to clean it out. Wow, he did a great job. Finally it was nice to look out back and see how neat and tidy it is as I had neglected it for years, just didn't have the time or energy :(

Now, regarding your Mom's headaches, this is the high pollen season and that can play havoc with ones head. And/or if your Mom wears glasses, might be time to get new lenses.

Fresh air, gentle exercise and the glories of nature are good for most people. And many people do find pleasure and relaxation in gardening. But you know your mother: has she ever shown any interest in these things? Do you have any public gardens or gardening clubs near you that you could perhaps take her to for a taster?

I see that your profile explains that vision problems are your mother's main concern. Is that why you took her to the doctor? How old is your mother, and what are the worrying symptoms?

Dear Mary,

I think this is only one option. It never hurts to be outside enjoying the sunshine. There are so many ways to reduce stress. I think being in a garden is one of many ways. I know its expensive to renovate a backyard. Maybe even having some extra flowers or lawn ornaments might do the trick.