Hot and Cold!

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://​plan​thar​di​ness​.ars​.usda​.gov/​P​H​Z​MWeb/ 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.

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. 

The new 2012 map is based on 30 years of tem­per­a­ture data (19762005) 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.