The area of a circle with respect to its diameter is:
A=(πd^2)/4
If he plants roses in every other section, there will be half of the area remaining, so half of the area of the circle is:
A=(πd^2)/8, since d=8:
A=(64π)/8 m^2
A=8π m^2 and if you want an approximation:
A≈25.13 m^2 (to nearest hundredth of a square meter)