I grew a 9 foot sungold last year in a 30 inch pot on my deck(thats in a 3.5 month season near Edmonton, Canada). you should be able to do better in your area with your longer season. if you want a big plant though, you need a big pot. I would say your 25 gallons should be ok, good compost ammended high quality planting mix, weekly fertilzer, strong support and consistent watering. you can grow a great one I bet. sungold is known for being a big and vigorous plant. not typically 22 feet by any means but big for sure. enjoy! send a pic of your giant sungold when you have one
KO