This one is definately worth bookmarking.

http://fr.cfans.umn.edu/extension/index.html

Here's just some of what it includes:

*Urban & Community Forestry (Landscape Trees)

**Tree of the month

**Seasonal Care for Trees & Shrubs in Northern U.S. climates

**Seasonal Topics: Why Leaves Change Color, Protecting Shrubs and Trees Against Winter Damage, Watering Trees

**Woodland Advisor Training

**Hot Topics: Emerald Ash Borer, Elm Replacement, Oak Wilt Advisory

**Contact Info: Need tree related assistance?

*Forest Management & Harvesting

*Agroforestry

*Tree Health

*Research

*More Links

See also the really cool interactive plant key!