The peak time for viewing fall colors in Iowa is on its way. (photo/Cindy Hadish)

An exceptionally warm and humid September delayed the arrival of fall colors in Iowa, but the Iowa Department of Natural Resources notes that leaves will soon be turning.

The week of Oct. 20 appears to be peak time for viewing fall colors in many locations, according to the DNR.

More: Popular route for viewing fall colors in Iowa

In spots in northeast Iowa, the DNR notes that hard maples are starting to turn to  yellow, orange and red. The aspen are now a brilliant yellow and some oak trees are beginning to turn golden brown.

Learn about fall colors in other parts of the state in the Weekly Fall Colors Report.

The Iowa DNR offers the following color guide to popular trees in the state:

Green ash leaves turn yellow, but white ash has a purplish cast. The leaves fall after those of walnut trees, but earlier than those of oaks and maples.

Elm leaves turn various shades of yellow with some turning brown before falling, others falling while still yellow.

Leaves turn yellow on hickory trees, then brown before falling.

Maple (Soft):
The leaves of soft (silver) maples turn yellow but do not turn brown before falling.

Maple (Hard):
Brilliant flame red hues are the signature of hard maple leaves. The red pigmentation of some leaves breaks down before falling.

Bur Oak:
Buff to yellow colors predominate in bur oaks. The leaves remain on the tree and turn brown before falling.

Oak (Red):
The red oaks have brilliant red leaves in fall though the color is probably not as intense as that of some hard maples.

Oak (White):
White oaks have a more subdued purple fall leaf color. The leaves then turn brown and often stay on the tree until new leaves begin to grow in the spring.

Related: Experts recommend these six trees to plant in the Midwest