I didn’t find proper explanation for this particular line. It is clear that she is comparing her in-law’s house with marigold. I google about marigold to know about its characteristics.
Marigold is an Inflorescence (group or cluster of flowers). Inflorescence, as in every petal of Marigold is a complete flower in itse lf. Even though each petal is a flower by itself, the beauty of Marigold lies only when they are all together. So, now the line “Sasural Gendha Phool” makes a lots of sense. Also this song beautifully justifies the joint family concept.
The song goes like this;
Saas gari deve
(she goes on explaining how she misses her husband when he is out for business and remembers their time together and how she admires him and his uniqueness)
The reason she compares in-law’s house with marigold is because, every person in a family is unique and complete in their own way even then they make a complete family only when they stay together. This can be applied to any family.
Though she is not completely happy with the way she is treated by her in-laws, she looks at it with optimism. Which is quite good and which can be found in most of the Indian women who to adjust to the new environment (read in-laws house) they move in.
This is a simple folk song which teaches us to look at problems with optimism.
And not to forget the music is by A.R Rahman 
Dear Maulee,
You are never late with sharing. I read your blog today about Genda Phool and loved it. How beautifully you have done the service to those who wants to know.
And with the knowing, i am connecting with this song so very much now.
Thanks once again.
Raj