Mmm… we’re flogging a dead horse, in my opinion. Either that, or this is a wind-up. 
Anyone who decides to ignore good advice - is free to do so.
I’ve know some who do/did just that. Some of them seem to “get away with murder” .
Some of them do come a cropper and take the folk who employ them down into the mire with them.
Personally, I never use or recommend anyone without a Siret & Ins for the work they will be undertaking.
My thoughts too Stella
There are lots of things in France that can seem overly complicated - applying to join the health service, filling in a tax return, getting a blood group card… However if you go with the flow and do it the French way then it’s usually easy. But if you dig your heels in and insist on doing it your way then chances are you will either end up in a mess, get hugely frustrated, or have to resign yourself to living in a british bubble.
I’m guessing that the clients you have lined up are british…
The area of France I’m going to is 5 mins from Geneva. I lived in Geneva for 2 years a few years ago.
I know people there and already have work waiting for me for next season. They know I’m very good at my job. It’s 3 American families and an estate that I’ll be maintaining.
I don’t understand why they would want me to have qualifications for a job I already have! It’s completely insane.
If they demand qualifications then I guess I won’t be able to go. Let’s wait and see.
I do appreciate all the help! Thanks
Who ever William’s clients might be he is not arriving until next year so the grass will be extremely long by then 
If you’re going to be employed by these families and their estate then why can’t you be an employee and the families sort out your tax, health care and legal status etc? Isn’t that what employers do? Presumably you have accommodation waiting for you.
This part of France is very international with the UN being so close. I guess I will find some British people.
I tell you one thing, the grass will be even longer if I need a qualification to mow it! 
I will live with my cousin. The work I have is 20hours a week, so I want to be self employed to get other work too.
Personally, I cannot see any problem with you contacting the Offices already recommended and going through the necessary paperwork/whatever.
Your (apparent) steadfast refusal to follow the French Rules does not bode well for you to continue living in France. Likewise Americans who break French Laws by employing unregistered labour .
But, we have thrashed this enough in my opinion. You go your way… 
The fact is that the UK is a very unregulated economy compared with France, and indeed most Continental countries. This has both advantages and disadvantages, winners and losers - but it does come as a shock if you’re used to the ‘anglo-saxon’ approach. (I heard a discussion on French radio recently in which the term ‘anglo-saxon’ was used virtually as a synonym for ‘neo-liberalism’ - the unrestrained marketisation of all aspects of life.)
On balance I now prefer the French approach, for many of the reasons others have put forward here, but also because I have seen the ease with which anybody can set up in business in the UK without knowing what they are doing, or looking after their staff - leading to them undercutting competition in a way that is not sustainable, but before going down themselves they tear up ‘the going rate’ for others that are running good sustainable businesses, paying decent wages, etc.
But the fact that on balance the Continental system works better does not mean that it has no downside - and I can quite see that a gardening micro-business coming to France from a more ‘anglo-saxon’ economy will see it as unreasonable.
“They” won’t demand qualifications if by they you mean the American clients. They won’t care two hoots as long as you do the job for the price agreed, and they won’t hugely care if you are set up legally or not as it’s you that will get into trouble not them.
The issue is about establishing yourself legally as a self employed artisan in France so that you can develop and grow a business. It could be that your experience will be accepted by MSA in lieu of qualifications, but you don’t know until you ask!
Did you read the links provided - theMSA link that Tim provided shows how you can establish yourself as proper gardening business. Or the second link Stella provided shows how you can be a jobbing gardener for individual clients without any qualifications. There are restrictions to this second route so read it carefully - for example you can’t work for businesses.
(If you have agreed this work, how did you set the price? As in general you have to charge more here to cover the costs of social contributions. )
Thanks everyone for your help…
I feel confident I can get this sorted now.
Roll on next year.
All the best