Have you noticed how warm it's been lately? Your trees probably have.

The slow onset of fall color this season may be in part due to the unusually warm September across much of the Northland.

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With most days in the 70s and nights in the 50s, Duluth temperatures have averaged more than four degrees above normal so far in September, and that may be tricking some trees into thinking it's still summer.

It's been a good summer for trees, with ample warm days and rain, and that should mean vibrant colors when they do come, said Eric Singsaas, director of the Wood and Bioeconomy Initiative at the University of Minnesota Duluth's Natural Resources Research Institute. Trees this year are generally in good shape, unlike in drought years when stressed trees may lose their leaves early without turning bright colors. (Very early coloring and drying are signs of unhealthy trees, not an early autumn.)

Experts agree that the onset of fall color is triggered by shortening daylight hours and the impact that change has on the chemistry of leaves. But temperature also plays a role, and cooler nights help bring out more color.

"It's that combination of shorter days, but especially the colder temperatures, that cause the change in hormones in the leaf that trigger senescence, the color change,'' Singsaas said. "The warmth we've had is slowing this all down."

Val Cervenka, Forest Health Program coordinator for the Minnesota Department of Natural Resources in St. Paul, agreed.

"If it stays warm too long, the color is not going to be as brilliant," she said. "Light frost and cool nights in the 40s really helps increase that color."

So far, there have been few cold snaps, and, except for parts of the Iron Range one day last week, no frost in most parts of the region.

In the next few weeks, weather from day-to-day plays a key role in how vibrant the leaf colors get and how long they last.

If it gets cold too fast, with hard frosts or a deep freeze, colors can fade quickly. Dry periods even this late in the growing season can lessen colors. Heavy rain and strong wind quickly strip trees of their leaves, cutting the color show short.

Some colors already are showing across parts of Minnesota and Wisconsin, and not always in the usual north-to-south pattern. While much of Minnesota is at less than 25 percent of peak color, some areas of central Minnesota are reporting color a bit earlier than usual, including Charles Lindbergh State Park near Little Falls which on Tuesday reported 50-75 percent of peak color. Lake Bronson State Park in northwestern Minnesota is reporting about 50 percent peak.

Maples on the back side of Lake Superior's North Shore hills - from Duluth to Grand Portage - will peak over the next two weeks, with aspen to follow. Trees right along the shore, however, may not peak until mid-October.

"They just really started to go this week. I'd say we have about another week until peak color," said Michael Baker, owner of Wild Country Maple Syrup in a maple-saturated patch of woods north of Lutsen. "I'd say it usually peaks around the 23rd to the 26th, up here away from the lake."

Trees in southern Minnesota won't see peak color until later in October.

Cervenka said the far southwest corner of the state is the only place where conditions are too dry.

"Otherwise, we've got an overabundance of water everywhere, so it should be a great year," Cervenka said. "We're going in with really healthy trees, which gives the trees the best chance of showing fall color."

Chlorophyll gives leaves their green color, while anthocyanins, carotenoids and tannins produce the reds, golds and browns that come to the forefront in the fall.

If you are looking for a new fall color experience, the DNR is suggesting Northeastern Minnesota's ample Scientific and Natural Areas which include unique geologic and natural features in addition to colorful trees. But you'll have to get out of your car - most all of them are walk-in only.

Some standout areas include:

• Sugarloaf Point near Schroeder: A "world-class example of fluid basalt lava flows from the Precambrian age." Located next to the Sugarloaf North Shore Stewardship Association with interpretive center, trails and kiosks.

• Iona's Beach, near Castle Danger: The unique natural beach is comprised of pink rhyolite slabs, smoothed by wave action into flattened, shingle-like pebbles. As the waves recede, the shingles come to rest with a tinkling sound unique to this site.

• Spring Beauty Northern Hardwoods, near Hovland: At more than 400 acres, this SNA showcases a continuous canopy of old-growth sugar maple along the northern edge of its normal range.

• Hovland Woods, near Hovland: Located within the Grand Portage State Forest, the wooded location contains mature and old-growth virgin forest communities now rare in the region.

• Lutsen, near Lutsen. One of the largest upland old-growth hardwood acreages along the North Shore, where Eagle Mountain, Raven Ridge and major ridges of the Sawtooth Mountains rise more than 800 feet above Lake Superior.

• Purvis Lake - Ober Foundation, near Ely: Dominated by large white and red pines, the topography of this site reflects the effects of numerous glacial advances, with alternating lakes, bogs and rocky ridges. No maintained trails within this location.

• Lost 40, near Alvwood: Spared from logging by a surveying error, the virgin old-growth, white pine and red pine forest is considered to be the most significant stand outside of the Boundary Waters and Itasca State Park. Minnesota's state red pine "Big Tree Champion" is found here and is 120 feet tall with a circumference of 115 inches.

For more information go to www.mndnr.gov/snas. Picking or collecting plants, rocks and other natural elements is prohibited at most SNAs. Visitors are advised to wear blaze orange during hunting seasons at those SNAs where hunting is allowed.

To learn more

Get up-to-date reports on fall colors at dnr.state.mn.us/fall_colors and at travelwisconsin.com/fall-color-report.

Forum News Service contributed to this story.