Above, from left to right: Forced bulbs in coldframes at The Cloisters; Crocus chrysanthus ‘Cream Beauty’; Narcissus tazetta ‘Inbal,’ (paperwhites) in Cuxa Cloister.
Every year, by the time February approaches I have the winter doldrums and the beauty of spring is long forgotten. The days become shorter, darker, and extremely colder. It is at this time that my senses need to be reminded why I ever decided to become a gardener. Forced spring bulbs during the winter months provide that reminder wonderfully. Just when I think there is no possible way to endure another day of winter, the first pot of forced paperwhites, Narcissus tazetta, is unveiled at The Cloisters.
“Forcing” is the term used to describe the treatment of stimulating plant growth out of season, under artificial conditions. In a way, forcing speeds up the normal flowering process that nature provides. This practice is most commonly and easily done with spring bulbs. Bulbs have great capacity for food and energy storage. These energy reserves make it possible for spring bulbs to be forced to flower at any time of year. Forcing refers strictly to the process of stimulating flowering. This process is normally preceded by a chilling period, which mimics the temperatures a bulb would experience naturally during the fall and winter months. It is during this period that root development and early stem elongation occurs. This is a very important part of the process. Do not be tempted to shorten this period for convenience; it will greatly compromise the quality of the finished plant. The length and temperature of the chilling period vary with the type of bulb (twelve to fifteen weeks). The required temperature is normally 35°F to 50°F. Temperatures should not go near freezing or above 55°F. Chilling and forcing times should be supplied from your bulb source. Comprehensive bulb forcing schedules for all species can be found online.
Once root development is obvious, pots should be removed from cold storage and placed in warmth and light, where leaf and flower formation will occur. A soil temperature of 55°F to 65°F is ideal. The length of time varies with the bulb, but flower forcing normally takes two to four weeks. Grow lights or a sunny window will work. Once flower buds are formed, plants should be removed from heat and direct light to extend the bloom time. (Bryan John, The New Royal Horticultural Society Manual of Bulbs, 1995).
This year we have developed a forcing schedule that will provide us with flowers from mid-December until early spring. This year’s bulbs include the medieval species Narcissus pseudonarcissus ssp. obvallaris (daffodils), Fritillaria meleagris (checkered lilies), and Ornithogalum umbellatum (Star of Bethlehem); and the non-medieval species Narcissus tazzetta ‘Inbal’ (paperwhites), Hyacinthus orientalis ‘Festival Blue’ and ‘Festival White,’ and Crocus chrysanthus ‘Cream Beauty’ and ‘Ruby Giant’. The bulbs that require a chilling period were potted up in assorted-size plastic pots. These pots were grouped by species and placed in coldframes that were covered with twelve inches of perlite. The perlite serves as insulation to prevent freezing. The pots will be dug out at different times, depending on the required chilling. A map of the precise location of each pot in the coldframe is created. Once chilling is completed, we bring the pots inside where they are placed under high-pressure sodium light bulbs. The soil temperature is between 60°F and 65°F. Once the flower buds have formed, the plants are placed in terra rossa clay pots and the soil is covered with moss. At this point the plants are ready to go on display and contribute to the famous beauty of The Cloisters.