There is nothing you can do about this problem, unfortunately. This plant's alpha map has jagged edges, so your only option is to create another alpha for it yourself. then you'll notice that the colour bitmap is of a very low resolution as well. So this plant cannot look good in close-ups. It looks fantastic in mid and background, but awful for a close-up shot. Same goes for all plants that use the same bitmaps ( date palm, and small palm)
Nothing to do with bad software, just bad images. This tree was there from version1 I think, and was never changed. A shame.