that happens usually for a number of reasons.
And for a betta tank it would be:
The plant is a cold water plant
The tank is too clean and leaves no nutrients for the plants
There is not enough lighting for the plant
The plant was planted wrong (ie for anubias an javafern which should not be buried)
The plant is not a true aquatic plant (ie lucky bamboo, peace lily etc)
but generally if you go for lowlight easy plants like anubias, crypts, java fern, Java moss, Susswassertang, elodea etc, you should be fine. acclimatize plants as you would if they were fish :)
duckweed is an easy plant, but it is also a pain to eliminate.
with lights, 6500k+ gives good plant growth
Last edited by ao; 07-12-2012 at 10:26 AM.