New film 'grows' into mini-greenhouse

A new product on the market helps to lengthen the gardening season by producing the sort of climate that plants enjoy even when nature didn't plan it that way.

The Swiss, in fact, invented it, while the British, who tested it extensively , report on it in very positive terms.

For several years now products have been regularly introduced that help take the bite out of spring and some of the frost from fall. For the most part these have been mini-greenhouse structures - tentlike covers of clear plastic that cover individual rows or beds. They work well; but while they keep out the cold, they also exclude the rain.

Now the Swiss have met the need with a product that lets in the rain or hose water while still surrounding the plants in a pleasing microclimate. It lies flat on the soil and expands to accommodate the growing plants.

The Fabro growing film, as it is called, is a sheet of clear plastic with a series of slits cut so that moisture can penetrate to the soil. As crops grow they push up on the plastic. The pressure opens up the slits and allows the film to expand. The gradually widening slits allow for increased moisture and air penetration while still maintaining temperatures and humidity above those of the outside air.

In the British tests the temperature beneath the film averaged 4 degrees F. above the outside air temperature.

This is how the Fabro film works:

Soil is prepared in the usual manner and seed sown (or seedlings set out). The film is then spread out over the bed and anchored with bricks or stones along either edge; or you can bury one side in the soil. Don't bury both sides because you may occasionally want to lift up one side and throw the film back so you can do some weeding.

Once the slits are fully open it is time to remove the film. By this time all danger of late-spring frost should be over.

If you are using the film to extend the season in the fall, be sure that the mature plants will not outgrow the expansion capacity of the film. Late lettuce would be no problem nor would carrots, the crop I used in testing the film last year. In my garden the carrots I planted in mid-September under the film grew somewhat faster and germination was better than in the carrot row left uncovered. The carrots also continued to grow when the unprotected carrots stopped growing altogether.

Such a planting is too late for large carrots in New England but we did harvest a worthwile quantity of fingerlings in early November.

Between March and December 1978 extensive tests of the Fabro growing film were made at the trial grounds at the Levington Research Station in Britain. The tests were done on many small-scale lots to duplicate as nearly as possible the conditions in the home garden.

The report summed up its findings with this paragraph:

''The Fabromont Expanding Cloche has improved germination and emergence times of sown crops, and has increased yields and advanced harvesting dates of many crops.

''Some damage from compression by the film was noted on crop varieties with a pointed growth habit if the film was left on too long. Increased damage by slugs was noted on some crops under the film. Timing and removal of this material from a crop is important.''

The report noted that all seed-sown crops emerged faster under the film and that 19 of the 23 crops tested gave improved yields. The four that gave lower yields were due to more slug damage and the fact that crops with a pointed growth were retarded by the pressure of the plastic once it had stretched to its limit.

As an example, two cabbage varieties were grown, one with a pointed head and one with round heads.

''The film was left on both crops for six weeks,'' the report said. ''The edge plants of the pointed variety became distorted by the weight of the film and there was a 10 percent reduction in the yield. However, the round-headed variety did not suffer any damage and gave a yield increase of 30 percent. Similar results were obtained with lettuce varieties.''

The Levington tests showed almost startling yield increases in beets and carrots (both up 50 percent), parsnips and one onion variety (up 30 percent), round-headed cabbage (20 percent), and one potato variety (10 percent). In all cases, including those where no increase in yield was evident, the plants matured ahead of the control varieties.

Two varieties of peas gave disastrous results, however. The test report says simply: ''Crop decimated by slugs.'' The researchers concluded that slug bait would have met the problem adequately.

It is possible that the more temperate English climate might benefit more from the Fabro film. My own limited experience with the film suggests its greatest contribution would be in speeding up the growth of hardy spring-grown plants here in the US.

Those interested in getting more details on the new grow film should contact: Garden Tech Ltd., PO Box 612, Needham, Mass. 02192.