I have a question about that pronunciation. "בוטן" is spelled with the vowel "segol" (the 3 dots) below the "tet". It is pronounced bo-ten. (Using English pronunciation of the "e" as in "bed".) When a plural ending is added to a word, I didn't think that the spelling or pronunciation of the beginning part of the word changed, unlike when a prefix is added to a word. It sounds to me like the pronunciation is bo-te-NIM, with the "te" being said very quickly. Can any of our Hebrew speaking site managers please weigh in on this question?
"When a plural ending is added to a word, I didn't think that the spelling or pronunciation of the beginning part of the word changed, unlike when a prefix is added to a word."
I can't answer your specific question about בוטנ/ים (I asked the original question!), but I do know that the pronunciation of the beginning part of a word can sometimes change a lot in Hebrew when a plural ending is added:
"book" ספר séfer > ספרים sfarim
"room" חדר khéder > חדרים khadarim
"shop" חנות khanut > חנויות khanuyot
"city" עיר ir > ערים arim
"pencil" עיפרון iparon > עפרונות efronot
Most of these changes are regular and make sense once you get used to the rules and learn enough Hebrew words that follow them, I've found.
There are historical reasons for this and other similar phenomena (see shwmae's excellent comment).
You can see the segol doing that often, it has developed in many places to replace a very short vowel which was apparently difficult to hold on to.
So yes, you should be aware that this exists. It can happen with any change - single vs. plural, conjugation, construct state etc.