The most common and most likely answer to this crossword clue is the 9 letter word ARBORETUM
A
R
B
O
R
E
T
U
M
More clues leading to the result ARBORETUM
- First of birches in mature or developing tree garden
- Run well within a corporation, there's high growth
- An initially rude, tiresome person’s to stomach botanical garden
- A right tedious type starts to tell us much in a site of botanical interest
- Perennial rings brought forth tons in e.g. Kew feature
- Adult tolerated temperature surrounded by odd collection of plants
- Flower beds carried first of tulips in botanical garden
- Where elders may be seen between services?
- Pal leaves perambulator around the trees
- Lily endured time inside botanic garden
Buddy explains!

The straight part
We know that "the trees" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case the trees = arboretum.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does ARBORETUM mean?
ARBORETUM noun- A place where many varieties of tree are grown for research, educational, and ornamental purposes.