2007-09 Freezing Studies
Low Perennial Groundcovers

Dr. Leonard Perry, Extension Professor
University of Vermont

 
 
These perennials have become popular in recent years, especially through several brands such as Steppables and Nooks and Crannies.  Most have been in the trade and are not new, just being marketed more widely.  Hardiness information for many has been spotty or often wide ranging, sometimes with as much as 3 or 4 hardiness zones.  In an attempt to gain a better understanding of hardiness of some of the most popular of these plants, the following controlled freezing studies were undertaken over a two-year period.
 
Methods
Plants were obtained in 3- or 4-inch pots, well-rooted, in late summer of either 2007 or 2008.  (In the table, those shown as "D" were obtained in 2008, the rest in 2007).  Plants were grown outdoors in Milton, Vermont until early November when they were brought into a greenhouse (polycarbonate with white poly cover), where they were maintained as close above freezing as possible (mean 38F, range 30 to 58F).  Plants were frozen in early January (dates and target temperatures in table).  Plants were frozen using standard protocol developed in previous studies-- held at 28F (2C) for 2 days in order for all pots to reach that temperature, then dropped to each of the target temperatures (over a 2 hour period) and held at each temperature for 30 minutes.  Plants were then removed from the chest freezers, returned to the greenhouse, and grown on for rating of survival and regrowth in spring.
 
Results
Of the 33 plants tested, 19 were found to be hardy, 6 moderately hardy, and 8 non-hardy. Groupings for hardiness are based on lowest temperature at which at least 4 (of 6 total per treatment) plants were living after winter.  Lowest temperatures are in ( ) at which plants rated 3 (marginally saleable) or better. Those with ( ) were only field tested.  Full details are in the table below.
 
In field trials, 23 of the 33 species and cultivars survived.  (Those with no rating shown for 2008 were only tested for one year over 2008-2009 winter.)   Most rated best (3) for regrowth and flowering (13), with others moderately acceptable (8) and just a couple barely living.  Several were rated better the first year, a few dying after the second winter.  This may be due to several factors, including growing season weather stresses (drought) the first full growing season or variable snow cover and winter temperatures.  Only a few showed no differences from the controlled freezing inside but rated less hardy in the field.  As in previous such studies, plants that are hardy under controlled conditions may be less hardy when in variable field conditions.  However, plants not hardy at lower temperatures under controlled conditions are generally not hardy outside either, as in this study.
 
 
Hardy (-11 or -14C, or rated 3 both years in field)

Achillea huteri (-14C)
Arabis ferdinandi-coburgi 'Old Gold' (-14C)
Armeria maritima 'Rubrifolia' (-14C)
Armeria maritima  'Victor Reiter' (-14C)
Campanula portenschlagiana 'Resholt Variety' (-14C)
Dianthus gratianopolitanus 'Tiny Rubies' (-14C)
Erigeron scopulinus (-14C)
Euonymus fortunei 'Kewensis' (-11C)
Gypsophila cerastoides (-14C)
Potentilla crantzii 'Pygmaea' (-14C)
Sedum requienii (-14C)
Silene alpestris (-14C)
Thymus pulegioides 'Archer's Gold' (-14C)
Veronica 'Mann's Variety' (-14C)
Veronica 'Waterperry Blue' (-11C)
Veronica allionii
Veronica liwanensis (-14C)
Veronica prostrata (rupestris) 'Heavenly Blue' (-14C)
Waldsteinia ternata

 
Moderately Hardy (-5 or -8C)

Lotus corniculatus 'Pleniflorus' (-11C)
Mazus reptans 'Motley' (-5C)
Sedum tetractinum 'Coral Reef' (-5C)
Veronica repens 'Sunshine' (-5C)
Viola labradorica

 
Not Hardy (-2C or above, or not hardy in field)

Anacyclus pyrethrum var. depressus 'Silver Kisses' (Silberkissen) (>-2C)
Artemisia viridis 'Tiny Green' (>-2C)
Erysimum kotschyanum (>-2C)
Houttuynia cordata 'Chameleon' (-2C)
Leptinella squalida 'Platt's Black' (-2C)
Mazus reptans (-8C)
Mazus reptans 'Albus' (-5C)