Save your plants for snails and slugs!
This ring is much stronger than most bought on Amazon, but still thin enough that you can print it for about 1 euro (less than 40g of filament).
There is a minor reinforcement at the part that you put in the ground, it will not bend excessively.
You can print it with an A1mini or bigger. I tested it on an A1 mini and X1C.
For best longevity, print with PETG or other outdoor suitable filament. Alternatively, use PLA (it prints faster, in about 40 minutes), but PLA will degrade faster in the sun.
You can print this in the fastest draft setting.