Undergravel Filter + Plants = ?

The roots of the plants will grow down and get all tangled and tied up in the grates. Can be a pain in the butt if you want to move any around. If you're not using the UGF why not just take out so you don't have those problems.

It won't have an effect on plants growing, but the roots can (potentially) grab the filter. I once had a tank with a RUGF set up, the plants did well. I had a deep enough substrate that the roots very rarely got tangled. I injected CO2 into the flow tube going down to the filter and it did an nice job dissolving it.

I ran a UGF/RUGF and the only issue was when I replaced the plates. The Crypts I had were the most tangled up, but I never noticed it causing any flow issues. Like mentioned, might become a PITA if you need to move the tank or redo the setup.