By Chris Erickson
Green roofs are extreme environments and present unique challenges in plant installation and care. Their limitations as habitat can sometimes put different horticultural best practices in conflict with each other. A time of year that this often occurs is in the fall. Because green roof installation is the last phase of building construction it is not uncommon to be planting in new roofs later in the year than would be ideal.
Fall is a great time to install new plants because the cool weather reduces transplant stress and facilitates root growth. However, late fall planting carries risks. If plants don’t have time to establish strong roots before the onset of freezing weather they are more susceptible to damage from exposure. On green roofs, where soil horizons are thin and wind speeds usually elevated, the risks of plants suffering damage from exposure is more acute.
Why are green roof soils so shallow? Weight limitations on roofs limit soil depth and so systems are designed to use as little soil as possible. On a Highview Creation’s roof in Brooklyn we maintain a variety of shrubs and a robust community of five-foot tall herbaceous perennials in only six inches of soil. This intentionally minimized soil layer can be a challenging winter environment for roots. Radiant heat from buildings can help to keep green roof soils warmer than their surroundings, but if the structure is not heated or a well-insulated roof traps this heat before it rises to the green roof, the thin profile of green roofs means their temperature will change more rapidly than soil in the ground.
A specific but common problem is that potted plants, grown by the nursery industry for planting in the ground where there are no space limitations, often don’t fit in green roof soil depths. To make plants fit in shallower soils it is sometimes necessary to cut off more of the root mass than would be ideal. A common workaround used to avoid aggressive root ball manipulation is using loose soil from the roof to create a mound that will cover the root ball. However, rooftops frequently have high winds which can erode away mounded soils and expose the roots before they can establish and hold soil in place. Roots that are out of the soil and exposed to cold winter air begin to dry out and can eventually suffer die back.
In green roofs with deeper soil profiles plants are at risk from the phenomenon of frost heaving. Frost heaving is caused by freezing and thawing cycles below the soil line. Water at a specific depth in the soil freezes, and in doing so expands and creates an ice layer. This causes the layer of soil, which now contains ice rather than water, to become extremely dry. Moisture from below is pulled into the dry soil by osmosis and the soil’s natural capillary action. As the rising moisture meets the ice layer it also freezes, thickening the ice layer. This process creates uplift pressure on the soil surface above.
When multiple cycles of freezing and thawing occur the result is a repeated lifting and settling of the top layer of soil which can slowly push buried items to the surface. The process can create cracks in roads, ruin house foundations, and disrupt plant roots. Plants that have a strong root system are typically unaffected, but new root balls can end up getting pushed out of the soil. Again, these exposed roots are at risk of desiccation and possible die-back.
Soils are at risk of frost heaving if they have at least 3% constituent particles that are .02 mm or smaller. Many green roof soils meet this criteria. When it comes to temperature swings, the other factor required to enable frost-heave, the freeze-thaw cycle on roofs can be, as discussed above, even more severe than what grade-level soils are subjected to.
There is an effective and simple option for reducing the risks of frost heaving, dry winter air, and freezing temperatures. This is to cover newly installed plants with a thick layer, often several inches deep, or not-yet-broken-down organic material such as leaves or mulch, which will trap air and insulate the soil. Unfortunately, organic material applications on green roofs must be done sparingly. Green roof soil is engineered to be low in organic material. The addition of organic matter can increase water retention in the soil itself which can lead to issues with weight load. More seriously, organic material can reduce or even clog the drainage systems underlying green roofs, leading to not just weight issues but roof leaks as well.
The best and most reliable solution is to properly time plant installation on green roofs. Early fall or early spring are best. When project timelines do not cooperate installing plants in a green roof is often a balancing act between the desired size of the plant specimens going in, the hardiness of the species selected, aggressiveness in root pruning, and the selective application of organic matter.