Spring Seeding Guide - Season Extension Without A Greenhouse:
(Just how big is that windowsill?)

You want to grow up transplants that are thick-stemmed and stocky. Plants that have too little light become leggy and fragile as they stretch upward toward the sun or another light source. For seedlings grown up in natural light in greenhouses or coldframes or hot frames, this is generally not an issue. For plants germinated inside a house, lack of light makes ‘legginess’ a given unless supplemental light is provided.

The usual, well-proven arrangement makes use of the 4’ shop brackets that hold 2 x 40 watt fluorescent tubes – standard tubes suffice, a combo of cool white and warm white is best. They work just fine for the first 4-6 weeks of a plant’s life. Plants need a period of darkness each day. The pros suggest 14-16 hours of light is optimal, using a timer to keep things regular. Ambitious space hogs will run two different sets of seedlings for 12 hours each day. By no means optimal, but give it a go if you must.

Beware of graphics (in every gardening book I have encountered) which show fluorescent lights hanging a foot or so above the seedlings. The light tubes need to be as close as possible to the plants, short of touching the leaves to the glass. Four to six tubes make growing up plants easier to manage (the strongly illuminated area is larger). At no juncture do you want the lights more than 3” – 4” above the plants. It is easier to lower flats than raise lights, especially when different plants get tall at different rates. Such lighting, in a warm basement, is entirely sufficient to grow up plants. Watering plants close to your partner’s favorite rug can get messy.

A robust, tried-and-tested approach involves using a heat mat under a flat, with supplemental lighting above. In my greenhouse, I have no supplemental lighting, but I do have a mini-cloche on top of one of my benches where the heat mat is situated. At night, I close the cloche up, keeping the heat mat on. During the colder nights of early spring I will also cover all my plants inside my greenhouse with ‘remay’, a light, white, translucent, sheet-like fabric which partially insulates the plants from plunging temperatures, lending a 2-3 degree additional window of protection. Occasionally, of a morning, I find the corner of a flat inadvertently left uncovered, frozen out, where the part of the flat covered by the sheet is fine.

Raising the Roof

One of the most sophisticated season-extension tools available to a home-gardener is a hot frame (a traditional cold-frame – essentially a large box on the ground with a glass lid – supplied with extra warmth). In some key respects (such as holding off the effects of frosts, for example) tiny-volume hotframes actually have advantages over large-volume greenhouses. With ready, healthy access to outside light and ventilation, but with the addition of a heat mat, early Spring germination and growth rates pick up markedly.

The most pressing management issue surrounding hot frames relates to ventilation. A sunny spell will turn a closed-up frame into an oven within mere minutes. You can open and close the lid manually with a reliable prop stick, in response to the daily weather, or remove the chore completely by installing an automatic opener to the frame window. These are designed expressly for the purpose, run without external power, and lift and lower the window as temperature demands.

A 3’ x 6’ frame is spacious enough to grow up a wide range of transplants for a large family garden. They are simple to make: construction plans are commonplace. The season-extending strengths of a heat mat/automatic opener combo, in a hot frame, cannot be understated.

Recommended reading: “Gardening Under Cover” by William Head, Sasquatch Books, 1989.



February 8,2008

By Nick Routledge for the Observer Allotment Blog