Am I allowed to bring Bonsai trees back

It might be a silly qustion but my mum asked me to and I dont know.

Am I allowed to ?

Depends on the customs rules of your home country. I think that most countries do not allow travelers to import soil and plants.

Yes, that is a question for the Customs department of your own country. In New Zealand for example it would be an actual crime to do so as the plant or soils may contain insects that would damage native plants or diseases that would infect livestock, like foot-and-mouth, but your country may have different regulations.

If you are from the UK I think the answer is probably no, as island countries tend to be the strictest on biosecurity.

so when i fly out, i can ask that time.

stupid question, can I send it via the post office

As above

If you can't bring it in through regular airport customs, I doubt they would allow it through the post either, as customs goes through the post as well.

UK Customs:
http://customs.hmrc.gov.uk/channelsPortalWebApp/channelsPort...

Restricted goods
certain plants and their produce, including trees, shrubs, potatoes, certain fruit, bulbs and seeds. For advice call DEFRA Plant Health, 08459 335577

It doesn't make any difference how it comes into the country, by plane, post or ship, if the item is restricted it isn't allowed in. Posting it won't remove any insects or diseases that might be hiding in the plant or soil!

as above

They --plant nurseries and plant shops--sell bonsai in continental Europe so they likely sell them also in the UK.

I live in the UK and you can buy the bonsai's here at various shops, I think they even sell them at M&S.