FROM THE BOARD 



Overwintering Containerized Perennials 



Overwintering containerized 

 perennials is probably the largest 

 risk that a perennial grower faces. 

 I think that more plants are lost 

 during overwintering than during 

 any other time in the season. Some 

 plants just don't overwinter well in 

 containers. Others do fine one year, 

 than poorly the next, for no appar- 

 ent reason. This makes identifying 

 the problem much more difficult. 



As fall approaches, I consider six 

 factors which I feel are the most 

 crucial to overwintering: 



1) General plant health going 

 into the winter. 



2) Excessive cold. 



3) Excessive heat 



4) Soil moisture. 



5) Disease problems. 



6) Rodent problems. 



V The health and well-being of the 

 plant is the single most important 

 aspect of its wintering successfully. 

 An unhealthy plant probably won't 

 overwinter no matter what you do 

 to it. Tied in with the actual health 

 of the plant is its condition in the 

 poL Some plants, (Alchemilla and 

 Heuchera for example), overwinter 

 very poorly if they are pot-bound, 

 or more specifically, if their dor- 

 mant eyes are crowded. Conversely, 

 unestablished plants generally don't 

 overwinter well. If we are potting 

 up plugs from seed or cuttings, we 

 like to get them into their perma- 

 nent container in August or the 

 first two weeks in September. After 

 that, the chance of the plant's root- 

 ing-in diminishes, and they have a 

 much harder time overwintering. 

 Bare-root divisions with a hard 

 crown (Hostas and Astilbes) can be 

 planted into October and survive 

 pretty well, but they won't be root- 

 ed in for early spring sale. Bare- 

 root perennials with loose crowns 

 (Achillea or Asters) should be pot- 

 ted up by mid-September. 



Peter van Berkum 



2) TTte main point in overwintering 

 hardy plants is protecting them from 

 extreme cold temperature. Root-kill- 

 ing temperature varies from plant 

 to plant, but for plants hardy in this 

 area, when the soil temperature 

 gets below 20 degrees P., root dam- 

 age becomes a threat 



There are generally four ways to 

 overwinter perennials in containers: 



1) Under foam blanket. 



(ie. polyfoam or microfoam) 



2) Under straw. 



3) In unhealed polyhouses, under 

 foam blankets. 



4) In polyhouses at low tempera- 

 tures, (ie. 32 degrees F.) 



So far, we have done all of our 

 overwintering under foam blankets. 

 We find it satisfactory, but not 

 ideal This entails putting all our 

 plants j)ot-to-pot in beds that corre- 

 spond wth the size of the foam 

 that we use, rolling the foam over 

 it, rolling white polyethylene sheets 

 over this, and weighting town the 

 edges with the windiest day of the 

 winter in mind. Good objects for 

 weighting the blankets down in- 

 clude concrete blocks, old tires, 

 logs, etc. A trick that we find help- 

 ful is to try to create troughs be- 

 tween the beds to hold snow and 

 rain. When it freezes up it is the 

 only secure means of holding the 

 blankets down in high winds. We 

 prefer to use the blankets that are 

 12 feet wide so we don't have to 

 overlap narrower (6 ft. wide) blan- 

 kets to cover the beds. Be sure 

 that your beds are 3 to 4 feet nar- 

 rower than your blankets so you 

 have adequate material to hold 

 down. 



For a nursery trying to overwin- 

 ter a small number of perennials, 

 straw or salt-marsh hay would be 

 ideal Just sandwch 8 to 10 inches 

 of straw between two layer of white 

 poly and you have a good blanket 

 But on a large scale, the time 



needed to apply it and the question 

 of what to do with the straw in the 

 spring makes it impractical. 



Unhealed poly houses are get- 

 ting much more use now. This en- 

 tails narrow hoop houses (12 to 17 

 feet is usual) with white poly on 

 them. Inside, the plants are set out 

 just like before with the foam blan- 

 kets. However the blankets don't 

 have to be weighed down. Perhaps 

 the biggest benefit to this is that 

 there is air circulation under the 

 blankets. This will be addressed in 

 the disease discussion. University 

 tests have found this method to be 

 slightly more effective than straight 

 blankets, perhaps giving a 1 or 2 

 percent better survival rate. 



Overwintering in poly houses at 

 minimal temperature, usually 32 de- 

 grees, can be effective, but the cost 

 of heating the greenhouses makes 

 it uneconomical. 



3) Excessive heat build up brings up 

 the question of when to cover and 

 uncover the plants. This is the hard- 

 est decision of the whole process. 

 If the plants are covered too soon, 

 heat can build up under the blan- 

 kets and damage the plants. If they 

 are left on too long in the spring, 

 heat buildup will cause the plants 

 to put on a soft flush of growth un- 

 der the blanket Then, when you do 

 uncover, this growth will be tender 

 and will be killed by any freezing 

 temperatures. (This soft growth can 

 be hardened off by uncovering be- 

 fore 3 or 4 days of cloudy frostless 

 weather or by rolling the blankets 

 back over the plants before the 

 nightly frosts for several days. This 

 can be an immense task, depending 

 on the size of your nursery.) What 

 makes this all complicated is that if 

 you wait too late in the fall or un- 

 cover too early in the spring, you 

 run the risk of severe low tempera- 

 tures that could cause root damage 

 to the plants. 



February & March 1993 

 3 



