That would work if you liked that look. But you also want to think about went the plant outgrows the pot. Changing to a hang on the back filter would probably be the best choice in the long run as it will give you more flexibility.
Undergravel filters are not very good and can cause OTS, I would throw out the idea of having a UGF altogether in a planted tank. Even if you did get it to work, it be one of the worst filter choices. ORUGF are better but with a small planted tank your best best is going to be a small Canister (no surface agistation to outgas CO2).
With plants your opening a whole new can of worms.
You can either go with Low Light plants, and something like excel as your carbon source; or high light + CO2 Injection; DIY would be fine on such a small tank. With Co2 and high light you also need to dose nutrients, Macros and Micros. Macros are Nitrates and Phosphates K etc... Micros I have been using regular Flourish.
You might be able to get by with high light and flourish excel as your carbon source, but DIY is not very hard, and does not rewquire dosing excel every day, although you can if you want! you could even buy the Hagen CO2 system if you don't want to make it yourself (Soda bottle with tube sealed in the cap). Plant to spend some money even on a small planted tank.
Now if you go high light without CO2 you will grow algae, if you go without nutriets you will get Algae and or problems with your plants. The plants need to outcompete Algae, and they need these thigns to grow properly otherwise algae willt ake over. To an extent it is all or nothing... Just be prepaired. Also I would invest in a plant substrate so you don't have to change it later.
Don't expect most plants to grow with a standard aquarium light and a regular substrate.
If your going high light, get some fast growers like Sunset hygro, Wisteria, and some easy growing lower light plants like Crypts, and Java fern to start with.
Co2 Injection sticky is a great place to start if you don't want to grow algae rather than plants.
Java fern & anubias can be tied on to a rock or piece of wood instead of planted in the gravel. Those are easy to grow in even low light with any filtration & easy to move that way. Then when you decide you want more,different kinds of plants you'll need to make changes-- bigger tank, more lighting etc. Many people (not me) grow fantastic aquascapes in very small tanks but they take lots of care, I suggest working up to it more slowly if this is your first try with live plants.
Nancy
Do a search on this forum for posts by RTR and UGF and RUGF filters. You will find that many have successfully kept planted tanks with undergravel filters. There is no reason not too if you allready have the setup. Plant roots can't tangle around the filter since they filter is simply a plate covered by garvel. The gravel bed is the filter. Does gunk accumulate? In a UGF it does but a RUGF doesn't. Still a decent gravel vacuum will take care of things.
I have grown plants before with an UGF. It's not difficult or really any different from a tank without a UGF. The two things I did run into was if the tank was too heavily planted it was difficult to vacuum the gravel which is necessary to maintain the UGF, and some roots would tangle on the filter plate meaning you had to be very careful when moving plants.
However I've found that I prefer to grow plants in a tank without a UGF. I also tried the reverse flow setup for a while and it worked too. RTR was a big help with that.