Homegrown National Park
The website of Doug Tallamy, a professor in the Department of Entomology and Wildlife Ecology at the University of Delaware and author of two very readable and important books - Nature's Best Hope - A New Approach To Conservation That Starts In Your Yard (2020) and the earlier Bringing Nature Home: How You Can Sustain Wildlife with Native Plants, both from Timber Press.

Tallamy’s Most Valuable Native Plant Genera
Tallamy's 20 most valuable woody and perennial native plant genera in terms of supporting biodiversity in the mid-Atlantic region.

Native Plant Finder
Tallamy has since zeroed in on keystone native plants. Here's the guide. It's US-oriented, so for Ontario, pick a random zip code from New York State or Michigan, which will get you the genera; for the species, consult an Ontario native plant purveyor.

Ontario Wildflowers
Just the best for identifying native (and non-native) wildflowers. Related sites cover trees and shrubs, ferns, grasses and more.

Monarch Watch
So much useful information about how to assist in the survival of this beleaguered butterfly which migrates from one end of the continent to the other, battling loss loss of food plants and lack of host plants for caterpillars all the way. Get a Monarch Waystation sign to alert the monarchs to all the goodies you've planted for them. Click on the blog for the latest from expert Chip Taylor.

Scorpionfly
Well-organized, well-illustrated information on wildlife gardening, the role of native plants and the communities of which they are a part. Which plant will attract a specific butterfly or bird? How to build a brush pile? What works best in a container? This is the site with the answers.

Little Forests
Based on the principles espoused by Japanese botanist Akira Miyawaki, this is an initiative by the Rideau 1000 Islands Master Gardeners. There's a wealth of information about planting Little Forests. "Forests are so much more than a collection of trees. Forests are complex multilayer plant communities. Forests are webs of relationships between plants, animals, fungi, birds, insects, bacteria and humans."

Ontario Nature 
An important organization that protects and restores natural habitats through research, education and conservation, connecting thousands of individuals and communities across the province. Publishes ON Nature, an excellent magazine that is worth the price of membership.

High Park
High Park is the best remaining natural area on the City of Toronto’s Iroquois Sand Plain.

Ontario Invasive Plant Council
The Ontario Invasive Plant Council has been inactive due to Ontario government budget cuts in 2019. This year (2021), the AGM is being held in January, stay tuned on what the future holds.

Host Plants for Butterfly Caterpillars
While most butterflies can feed at most nectar plants, native and non-native alike, their caterpillars have very specific requirements for the foliage that they have evolved to digest. Here's a good list, with precautionary warnings, from Garden Web. 
Andy's Northern Ontario Wildflowers
Keenly observed, beautifully photographed from the Sudbury area.

Illinois Wildflowers
Many of our native plants are also native in this Great Lakes state. Worthwhile and detailed descriptions

Bill Shotyk's Elmvale farm 
Professor William Shotyk of the U of A is my neighbour and the scientist who found that the groundwater in the Site 41 vicinity is purer than Arctic ice - but that's another story. This is about his passion for planting trees - thousands of them

A Tree Grows in Essa
AWARE Essa's tree project. Favourite trees, book reviews, gallery and more....

Growing the Medicines
This page from the Six Nations Farmers Market and Garden website offers advice on growing and harvesting tobacco, sage, cedar and sweetgrass

Ojibway Prairie Complex
Ojibway Prairie Complex, located in Windsor, Ontario, is a collection of five closely-situated natural areas within a 10 minute drive from downtown 

Fletcher Wildlife Garden
The Ottawa Field-Naturalists’ Club develops and manages the Fletcher Wildlife Garden as a way to demonstrate to residents of the Ottawa area how to create or restore wildlife‐friendly habitat and gardens, emphasizing the use of plants native to the region.

Tall Grass Ontario
Tallgrass prairies and savannas are some of the most endangered ecological communities in Canada. Here's information on how to preserve and restore

Ontario Woodlot Association
Lots to learn from this group that caters to those with a few acres or more, but full of info for anyone with an interest in nature

Forest Gene Conservation Association
It's important to source seed from the zone where the tree is to grow. This non-profit promotes conserving genetic diversity in urban and rural landscapes in southern Ontario

The Green Pages - Watershed Stewardship Business Directory
Compiled by the Nottawasaga Valley Conservation Authority, offering A wealth of information and contacts from environmental, agriculture and forestry organizations to contractors and consultants. A chapter advises on native plants and invasive species, including what to plant for your conditions and which are the best species to host butterfly caterpillars or feed nectar seeking pollinators.