You can always dechlorinate or debrominate your spa by adding a reducing agent just before you dump the water. That will turn bromine into bromide salt or chlorine into chloride salt. Odds are that the chlorine or bromine levels in your spa aren't enough to cause problems for your plants. After all, most irrigation uses chlorinated (or chloraminated) water. Nevertheless, to remove bromine or chlorine, the easiest reducing agent to use is hydrogen peroxide as it will simply result in producing oxygen gas.
You use roughly the same volume of 3% hydrogen peroxide to dechlorinate as it takes 6% bleach to produce a Free Chlorine (FC) level. You can calculate the FC equivalent from a bromine level by dividing the bromine level by 2.25. So using the
PoolMath calculator and assuming a 350 gallon spa size, you can remove 4 ppm bromine which is 4/2.25 = 1.78 ppm FC by adding 1.3 fluid ounces of 3% hydrogen peroxide (same volume as 6% bleach needed to raise the FC by 1.78 ppm).
It's more likely your hedges or other plants will not like the salt level (salinity) of the spa though chloride may be worse than bromide. You can minimize this damage by watering the area to dilute the water that you dump from the spa. You can also not dump in the same place every time, though given that you are likely only dumping the spa water once every so many months, it's not a big deal.
