Hi, KJC welcome to our little community. From the best of my research this is what you can do to get a Tilia Astera:<<<<<SPOILER Breed a Baccatus Orchid with a Tilia Gladiatus or mutate the following (only if you have a lot of money though) Tilia Orchid, Baccatus Astera or Tilia Gladiatus.

If you will comb through the Plant Tycoon Guides section you will find lots of helpful and interesting information there. Have a Great gaming Day!