Hot and Cold!

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photo: Tim Schenck

Guest post by Kim Mathews, ASLA, Principal at Mathews Nielsen

The Hills open this summer and the 2016 season will include the final planting push for the Island’s diverse new landscapes.  Altogether Mathews Nielsen Landscape Architects will have planted 7,100 new trees, over 48,000 shrubs and 9,000 perennials.   An exciting summer and likely a hot one!  This apparent new summer norm has many people asking the designers at MNLA about climate change.

The 1990 USDA Plant Hardiness Zone map that landscape architects have used for years to understand where plants will grow best was updated in 2012. The new interactive map http://planthardiness.ars.usda.gov/PHZMWeb/ shows a clear trend of warmer minimum temperatures. Higher numbers mean warmer zones.  For the NYC region, the 2012 map shows a change from zones 6b or 7a to zones 7a to 7b.  Governors Island is currently mapped zone 7b or an average annual minimum winter temperature of 5 - 10 degrees F.

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Climate change is a game changer for many aspects of planting design.  Hotter temperatures will allow some new species to survive here, but new pests and diseases will not be far behind.  A range of climatic variables certainly need to be taken into consideration when dreaming about plants, but the key detrimental factor for any plant continues to be the cold weather, not the hot.  A sudden, out-of-season freeze or very low temperatures can kill a plant immediately.  Soil moisture and high winter winds also enter the picture, adding to the plant’s stress and ability to survive a cold snap.

For all these reasons, MNLA considered a broad range of plant species, but we did not lose sight of the minimum temperature ranges and the microclimate conditions on this windy island.  As an example, while designing the Hills, the team considered plants that have tolerance for maritime conditions including high winds.  In selecting the thousands of shrubs that are being planted on the Hills, we looked closely at slope and aspect (sun/shade) in addition to many design parameters (mature size, growth rate, form, salt tolerance, seasonality, maintenance, and availability).  A top pick, the Rhus aromatica ‘Low Gro’ (Fragrant Low Grow Sumac) is a versatile native shrub that we call the “workhorse” of the Hills.  It represents over 20% of the 18 varieties of the shrubs and is catalogued as zone 3 to zone 9 giving it a wide range of tolerance. 

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The new 2012 map is based on 30 years of temperature data (1976-2005) versus 12 years of data for the previous map.  This advance is important for home owners, horticulturalists and designers, but zone information continues to be just one of the pieces of the plant kingdom puzzle.