Please tell me what can grow under a big oak tree. The soil is shallow and much limestone in the area… grass doesn’t grow. Is there a tall perennial flower? Or ground cover with flowers?
Thank you for contacting the Toronto Master Gardeners with your inquiry.
Trying to grow grass or perennials under trees is problematic for a number of reasons. First, the area under the tree does not receive enough sunlight for plant material once the tree has leafed out. Secondly, the tree canopy prevents rainfall from reaching the ground and lastly, the tree’s roots compete with grass and other plants for water and nutrients.
That being said there are a number of things that you can do to increase your success. Oak trees have deep roots; as a result there is room between the top of the soil and their roots to grow perennial flowers. The best perennials for this area are ones that are adapted to dry shade. The Toronto Master Gardeners have a Garden Guide Perennials for Shade in Dry or Moist Areas: A Toronto Master Gardeners Guidewhich lists perennials that are suitable for the area.
You might also consider planting bulbs. Bulbs make an excellent choice since they will bloom before the trees leaf out.
Another possibility is native plants such as wild ginger or sweet woodruff. These plants often naturally grow as understory plants in woodland settings. The city of Guelph has an excellent website Native and Drought Tolerant Plants that gives a list of native plants capable of growing in dry shade.
When purchasing plants buy the smallest size available. Remember it is always easier to dig a smaller planting hole, which will minimize disturbance to the tree’s roots.
Before you begin your planting it is always best to amend the soil before planting by adding organic material such as compost. Once the area has been planted adding 2-3 inches of mulch will help to conserve moisture and will supress weed growth.
Lastly, you might find the follwing two websites of use: Perennial Flowering Plants for Under Oak Trees and Planting Under a Tree.