Sometimes you have to wait a while for the plant to become harvestable, this always happens when you start a new save file or visit a place for the first time. As the plants only grow when you are physically on the lot or the neighbourhood where they are growing. Some of them can take 5 full days to become harvestable, so maybe take a tent and camp fire with you to the alien world, linger a few days and see if they grow?
Otherwise you can get the Rare garden seeds pack. Only problem it’s at random what seeds you get, so could take a while to get the alien ones you’re after.
If not, there is the debug menu cheat? However this will deactivate trophies for the save file?