I am not sure if you have posted the numbers correctly.

Some thing does seem amiss about this problem, but if you assume certain things (such as, the last row neednt be completely filled with plants), I guess we can attempt solving it.

Here is my attempt at trying to solve it:

Well, if there are 180 plants and 40 plants per row. That means its 40 plants in the first 4 rows and 20 plants in the 5th row. (Simple arithmetic: 180 plants / 40 plants per row = 4.5 rows OR 4 full rows and 1 half row.)

Now, if the question says that if above was the case, the gardener would've needed 6 fewer rows. This imples that he has 6 more rows than he would've had if he planted the 180 plants such that 40 plants were present per row, which woukd be 5+6 = 11.

So the gardener has 11 rows in total at present.

Hope this helps but since Arithmetic isnt one of my strongest traits, please correct me if this is awfully wrong.

Cheers!