About ten years ago we planted a ball and burlap red maple (about ten feet tall at the time) in the corner of our garden. Since then, it has grown considerably and now nothing will grow underneath it. I have had blueberries, raspberries and black currents underneath for several years which have yielded no fruit and are not thriving.
I plan to build raised beds. How deep must the beds be for berries? What kind of liner do I need on the bottom of the bed? Tree roots invaded my composter and I have moved it farther away.
Thank you for contacting Toronto Master Gardeners.
There are two different Maples that are commonly referred to as red maples. There is the Acer rubrum which is a native maple with green leaves and there is the Acer platanoides ‘Crimson King’ which is a Norway maple with red leaves. The Norway maple, though sold by nurseries, is an invasive plant to our area.
If you have the Crimson King they are notorious for not having anything grow underneath them. They have large leaved dense canopies that allow only minimal water to get to the soil under the drip line. There may also be some properties of the tree that are allopathic meaning it actively discourages other plants from growing near it. People often have trouble even growing grass underneath them.
No matter which tree you own, growing plants underneath can be an issue. Anything growing where the roots are is competing for water and nutrients with the tree. As there is only a finite amount of water and nutrients neither plant will do as well as it would if on its own.
Tree roots do not grow down, they grow outwards from the tree and extend far past the drip line. The roots need oxygen to thrive. If you put a bed on top of roots you will deprive those roots of oxygen and could potentially damage or kill the roots. When choosing a location for your raised bed it would be best to be as far as possible from the tree.
Berries grow best in full sunlight in well drained soil. As your are under your tree they would not have this so they have been struggling. Your best bet is as you have suggested to move them away from the tree.
You have listed berries that have different needs to grow and prosper. The Raspberries like an alkaline soil to thrive and produce but the blueberries like an acidic soil. As the two plants prefer the opposite soil they will need to be in separate places. To create and maintain the acidic soil for thriving blueberries does take extra time and must continue to be adjusted and maintained each year. That same soil would be detrimental to raspberries and currents. Of the three fruits the current is the easiest and most forgiving to grow. Currents are not as specific about their needs and require less nutrients. I am including below links for each of these plants for you to gather more information on each one.
Building a raised bed is fairly straight forward. You should not need to line the bottom of the bed. You want the bed to drain properly. With the new soil sitting on top the soil below will slowly loosen up and help with nutrition and drainage for the plants. Do not add any gravel or anything else between the new soil and the ground below. Gravel at the bottom actually decreases drainage due to the principles of Osmosis.
The following answer from our library talks about raised bed sizes.
Information for growing berries: