Overwintering Herbaceous Perennials in Containers 



LINDA BILODEAU, PETER VAN BERKUM, AND PAUL FISHER 



I 



n northern nurseries, overwinter- 

 ing is a major limiting factor in 

 .plant production for growers of 

 containerized perennials. Finding the 

 most cost-effective way to prevent 

 winter injury and reduce losses is a 

 challenge. The ability of containerized 

 perennials to withstand cold tempera- 

 tures during overwintering is limited 

 and varies between species (lies and 

 Agnew, 1995). Inadequate winter 

 temperature control (extremes in 

 warm or cold temperatures) can cause 

 plant death or can delay spring re- 

 growth (Herrick and Perry, 1995). 

 Temperatures that are too warm dur- 

 ing late winter can also induce spin- 

 dly growth that make plants sensitive 

 to freezing temperatures during 

 spring. 



Profitability is the deciding factor 

 in choosing the overwintering method 

 that will ensure the highest plant sur- 

 vival. Ease of installation, cost of 

 supplies, labor, reusability, and effec- 

 tive protection all need to be consid- 

 ered. Structureless protective cover- 

 ings vary in price as well as effective- 

 ness and are easy to install and main- 

 tain (lies et al., 1993). An excellent 

 cost analysis of overwintering was un- 

 dertaken by Beattie (1986), who 

 found that for zone 5-6, the cost to 

 overwinter plants was $0.25/sq.ft. for 

 no cover (because of fixed costs such 

 as fungicides and pest bait blocks), 

 $0.31 /sq.ft. for one layer of micro- 

 foam, $0.60/sq.ft. for an unheated 

 greenhouse with a single layer of 

 plastic and microfoam over plants, 

 and $0.92/sq.ft. for a heated green- 

 house with a double layer of plastic 

 (1984 dollars). 



Research was conducted at the 

 Van Berkum Nursery in Deerfield, 



New Hampshire, during the 1997-98 

 and 1998-99 winters to compare the 

 effects of different combinations of 

 protective coverings, media types, and 

 pot sizes on survival of herbaceous 

 perennial crops in containers. The 

 objectives were to quantify (1) what 

 soil temperatures were encountered 

 under protected and unprotected en- 

 vironments; (2) how covering affected 

 survival and vigor of several herba- 

 ceous perennials; and (3) whether 

 media type or pot size affected plant 

 survival. 



Six species were used in this trial: 

 Aquilegia chrysantha, Asclepias tuberosa, 

 Dicentra eximia (1997 only), Gera- 

 nium endressii 'Wargrave Pink' (1997 

 only), Lobelia cardinalis, and Phlox 

 paniculata 'David.' Transplants of 

 each species were planted individually 

 into 2.5 qt. square plastic pots on 

 August 12, 1997 (1997 trial) or Sep- 

 tember 10, 1998 (1998 trial), with 

 ten plants for each combination of 

 covering, medium, and species. 



The majority of plants were grown 

 in Fafard 51 media (20% peat/20% 

 coarse perlite/60% pine bark). Some 

 perennials were also planted in Fafard 

 50 and 52, ProGro PX3, and blends 

 of Fafard 51 or ProGro PX3 with 

 perlite (1:1 or 5:1 ratio) that pro- 

 vided different ratios of peat, bark, 

 perlite, and vermiculite. In 1997, we 

 planted ten of each species into 

 Fafard 51 in a 4-inch diameter PVC 

 pipe, 1 1 inches deep, taped on the 

 bottom. 



Plants were overwintered from late 

 November until late March each year, 

 in each of the following growing con- 

 ditions: outside with no protection; 

 outside with single foam; outside 

 with single foam and fleece; inside a 



heated greenhouse (temperature main- 

 tained at a minimum of 32F); and 

 inside an unheated greenhouse with 

 unlaminated microfoam cover. Pro- 

 tective coverings used included single 

 laminated polyethylene foam (3/8" 

 thick), non-woven fleece, unlaminated 

 microfoam (used as a blanket inside 

 the unheated greenhouse), and white 

 polyethylene greenhouse covering (6 

 ml). 



Small Hobo^^ data loggers were 

 used to record air and soil tempera- 

 tures every 2 hours 1 5 minutes from 

 November until March under each 

 cover. Snow depth was recorded 

 weekly. Plants were moved from Van 

 Berkum Nursery to an unheated 

 greenhouse at UNH Woodman Hor- 

 ticultural Farm in early April each 

 year. In May and June, roots and 

 shoots of each individual plant were 

 inspected, and plants were rated us- 

 ing a three-point scale: l=dead, 2 = 

 stressed or weak (sufficient damage 

 to affect market value of the plant), 

 or 3 = healthy. 



RESULTS 



Effect of Coverings on Soil 



Temperature: 



Temperatures are summarized in 

 Table 1 and soil temperature data for 

 1997-98 is also shown in Figure 1. 

 Minimum air temperature was lower 

 than soil temperature in all the envi- 

 ronments (Table 1), and maximum 

 air temperature was higher than soil 

 temperatures. The daily range (mini- 

 mum to maximum) in temperature 

 was also more extreme for ambient 

 air than for soil temperatures in all 

 the environments. These results are to 

 be expected, given the insulation and 

 buffering to temperature change of 



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The Plantsman 



