JARS v54n4 - Overwintering Seedlings in Plastic Containers

Overwintering Seedlings in Plastic Containers
Dr. Mark Konrad
Sewickley, Pennsylvania

Overwintering seedlings is a subject always open to innovative ideas. One that I tried recently seems quite promising, especially if you do not have a greenhouse.

Plastic containers are used to hold a number of peat pots (2¼-inch size) with seedlings started indoors the previous fall. I have become quite enamored with the use of peat pots for the following reasons.

1.  They allow for maximum aeration which is a high priority for rhododendron culture.

2.  The moisture can be easily kept at an ideal level. The porosity of the pots helps prevent overwatering.

3.  If a small layer of medium is placed on the floor of the plastic container, the moisture control is further enhanced.

4.  They allow for easy transplanting to soil which has been a big plus for growing on and they rapidly biodegrade once in the soil. The fine roots do not readily penetrate the wall prior to transplanting.

5.  The cost is relatively cheap when the pots are bought in bulk. The results are well worth the expense.

The volume of the plastic container holding the peat pots will depend on the seedling size. A wide variety of containers can be purchased inexpensively at many discount department stores.

Plastic box for overwintering rhododendron seedlings
Plastic shheting on box for overwintering rhododendron

For summertime culture, clear plastic sheeting is laid lightly over the over the plastic containers. Clothespins are helpful in holding this in place. The containers should be placed in a shaded, sheltered area to avoid excessive heat and rain. The humidity is estimated to be around 80 percent, which helps reduce the amount of watering needed. Rain water is preferred. Under these conditions the medium should be horticulturally sterile. Currently, I like equal parts of perlite with a seedling mix composed of a fine grade of vermiculite and Canadian peat. A surface application of fertilizer for acid loving plants can be added as the plants grow larger.

For wintertime care, the plastic sheeting is left in place or the lid for the plastic container can be put on. As freezing weather approaches, the containers are placed in an unheated garage. When rhododendrons are in dormancy, little if any light is necessary. When winter is over, the containers can be returned to the shaded outdoor area.

Sometimes, when seedlings are too small at the end of the summer season, an alternative method can be used. The seedlings are transplanted into 9-inch pots containing 3 inches of medium. Plastic paneling is easily used as a cover for the same unheated garage storage. Transplanting to peat pots can take place the following spring.

In summary, the advantages are these:

1.  Easier maintenance.

2.  Less vulnerability to insects and animals.

3.  Improved winter protection.

4.  Easier transfer to soil conditions.


It is interesting to note at this time that both David Leach and Guy Nearing used convenient, low maintenance methods of overwintering their seedlings. David Leach placed his flats of seedlings in an unheated, unventilated, totally dark shed. After the medium had frozen in December, they were allowed to remain that way until spring. He felt that dormant rhododendrons needed neither light nor ventilation. The seedlings were dusted with a pesticide prior to storage ( Rhododendrons of the World , page 359).

Guy Nearing used an unheated, narrow, walk-in pit glasshouse. This probably needed more maintenance but it was still very convenient.

Dr. Konrad, a member of the Great Lakes Chapter, is a frequent contributor to the Journal.