I am looking for a ground cover, drought tolerant, zone 3, that I can sow directly into grass that I hope the ground cover will eventually replace.
Thank you for contacting Toronto Master Gardeners.
As with any plant, there are many things to take into consideration when picking a ground cover.
You will need to take into account how much light the plants will get, as well as the quality and type of soil. Does your lawn get full, partial, or little sun during the day? Do you have a rich soil under the grass or is it low in organic content? Do you have good drainage or does it get boggy in the spring or extremely dry in the summer? All of these factors will need to be taken into account when choosing the best plants for your space.
Ground covers can be defined in many ways. Some are decorative while others can handle light to heavy traffic. One can cover an entire area with one type of plant or create spaces which use different plants creating a mosaic or design within the space. How the space will be used will probably guide this decision. If you have young children and/or dogs using the space the groundcover will need to be tougher.
When choosing plants it is good to be familiar with groundcovers that are considered invasive in Ontario. Unfortunately many invasive plants are still available in nurseries. Ground covers you will commonly see that should be avoided include periwinkle (myrtle, vinca), English ivy and goutweed. Once you have these plants in your yard they will spread rapidly and invade your gardens and your neighbors’ gardens.
Once you have chosen a plant you will need to prepare your space accordingly. You may need to amend the area with soil and compost. If you are planting plants you may need to clear areas and prepare them for the plants so they have a good chance to establish themselves before they begin spreading. Depending on your choice and the weather this summer, you may need to water more than you have in the past to get the plants well established.
I am including some links below for you that list many of the groundcovers that grow in Ontario. Many will only take light traffic. If your area has a lot of activity it may be easiest to use clover which will help fill in the lawn and put up with traffic. One can usually use seeds fairly easily and it will be fairly straight forward to seed within the existing grass. Thyme is another common solution. It is very resilient and certainly makes the yard smell wonderful. It is a little harder on bare feet than clover will be.
The important thing is to pick a groundcover suited to the area and the function of the space. This will allow you to enjoy the results with less work and frustration.
The following websites may be helpful to you:
https://guelph.ca/wp-content/uploads/HealthyLandscapes_Groundcovers_Lawn_Alternatives.pdf
https://www.invadingspecies.com/invaders/plants-terrestrial/invasive-ground-covers/