Green Roof Plants - What Makes Roots Happy?

Removing older green roofs to allow for a new project has opened my eyes to the way green roof plant roots grow.  Root growth science is also know as 'root architecture'.  

As you can see, roots do not like to be constrained.  Though all roofs have a perimeter and roots must eventually reach a boundary, allowing the plants to reach out for  nutrients and water offers many benefits!

Tomato plant roots reaching out horizontally in the Green Roof hurricane weave

Having grown plants in pots for decades I can say with no uncertainty root bound plants are prone to disease and do not preform well in the long term.

Pot bound roots spiral around and around looking for room to grow

Green Roof trays must be large to prevent root binding patterns

Normal root architecture is best achieved by open growing systems on Green Roofs

To me, a photo is worth a thousand words and it is easy for me to see how roots want to stretch out.  Of course we've discussed the many benefits of horizontal root architecture in previous posts.  An example  of a shallow green roof application can be found here.

Let your green roof roots grow.  If you use a tray system make sure the trays allow for root crossover through adequate openings.

Roots need oxygen too.  When roots grow too deep or are strangled by other roots due to a lack of horizontal growing room the plants can suffer.  Providing adequate horizontal growing room ensures good plant development, unless you are using tap root plants (not too many of these are appropriate for green roofs).