While there has been going on lots of discussions on the new sets for HotR, I think the discussions on the Ironblood set kind of overshadowed the Hagraven's Garden set.
It looks like a very cool and promising set, that is really unique from all other sets that exist so far. I really support sets that don't offer straight stats and introduce mechanics that one can build around. That said, the set has decent 2-4 pieces, with health recovery and healing taken that are usually used on other sets that have very strong 5 pieces to not make them overperform.
The 5-piece bonus is where the set is supposed to shine, offering a reservation area in which you take 50% less damage from sources that are outside of this area when falling below 50% health. Sadly, the area only lasts 5 seconds and has a 45 second cooldown. Additionaly, the first enemy who enters the area is knocked back.
Now I'm coming from the PvP side of the game and want to evaluate the set accordingly. In general, PvP is pretty fast paced with lots of dynamic movement, which generally makes stationary sets very weak. If one really wants to build around a stationary build, you need to heavily invest into defensive stats. The gardener set however doesn't offer much to compliment this playstyle, since you only have it up every 40 seconds which is a HUGE cooldown, and once its active you are forced to switch positions frequently anyways. Also I dislike the activation mechanics which kicks in when you drop below 50%.
My suggestions: Change the activation condition to a % chance when taking (ranged?) damage, similar to Ironblood. Regarding the insanley high cooldown, I'd suggest to increase the duration drastically to about 20 seconds and reduce the cooldown to 30 seconds. This still encourages for stationary playstyles while increasing the uptime.
As for PvE, I can't say much but from what I know the big damage for tanks usually comes from melee attacks anyways, and tanks are rather building into group utility than reducing their own damage taken. The 2-4 piece bonuses don't really play much into the PvE tank meta as well.
Conclusion: This is offers very interesting mechanics that I'd love to encounter in the next update, but it surely needs tweaking to become viable to use.