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Greenhouse Grower and Greenhouse Management and Pro-
<br />duction are periodicals that address plug production and re-
<br />lated issues. Greenhouse Grower produces an annual bonus
<br />issue that targets plugs and propagation. GrowerTalks holds
<br />an annual plug conference that lasts for three days, usually in
<br />October. Although the conference is expensive (usually
<br />around $550 for all three days), it would be worth the money
<br />to talk with growers from around the country to see what
<br />they recommend and discuss solutions to problems.
<br />Containers
<br />Plugs are produced in multi -cell plastic trays, also called
<br />flats. The trays are available in a wide range of cell sizes and
<br />depths — anywhere from 50 to 800 cells are present in a sin-
<br />gle tray. The type of flat chosen will depend on the species
<br />being grown and the length of time spent growing it. Pansies,
<br />for instance, are most often grown in 288, 392, or 406 plug
<br />trays. The time from sowing to ready- for - transplant stage is
<br />usually about 6 weeks in 288 trays and 5 weeks in 392 or
<br />406 plug trays. Larger cells provide more moisture and nutri-
<br />ents to the plant, but make much less efficient use of green-
<br />house space than the smaller cells.
<br />There are also rigid
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<br />trays, however, are
<br />quite bulky and will not nest for storage. Lightweight plastic
<br />trays, in contrast, are quite easy to store in the off - season,
<br />and cost less initially, but are less durable and usually require
<br />an under-tray to maintain sufficient strength for convenient
<br />handling. Heavier, thicker - walled plug trays are better suited
<br />to automated equipment. Winstrip® trays have small slits
<br />down two sides of each plug, to improve aeration. There are
<br />also holes between each cell to allow air movement up
<br />through the tray. Plugs grown in Winstrips usually have less
<br />root spiraling. Occasionally, growers choose a soil block sys-
<br />tem. Developed in Europe some years ago, soil blockers
<br />press a soil mix together into cubes that are handled in much
<br />the same manner, as peat pellets. Commercial -scale blocking
<br />equipment is quite expensive, and many growers find the
<br />plants' tendency to grow their roots into adjoining blocks
<br />somewhat annoying. Small -scale blocking equipment is also
<br />available and has proven popular with some market garden-
<br />ers. For more information on soil block methods, see the
<br />book The New Organic Grower by Eliot Coleman.
<br />New Research on Containers
<br />Research conducted at the University of Georgia during 1995
<br />showed that copper - treated plug flats affected the root
<br />gro%k1h and flowering of three species of bedding plants. Ar-
<br />mitagc and Gross reported that root spiraling and seedling
<br />height were reduced in Impatiens, Pelargonium, and Petunia.
<br />The copper treatment delayed flowering for the impatiens
<br />Page 7
<br />and petunias by twelve days and 21 days for geraniums, but did
<br />not affect their mature heights.
<br />Container size can affect transplant characteristics. Researchers
<br />at the University of Florida tested four sizes of Speedling flats
<br />and found that lettuce grown in the two largest sizes took
<br />longer to reach harvestable size, but yield was not affected. In
<br />the spring -grown lettuce, there was no difference in head
<br />weights, but fall -grown heads started -in the larger cell sizes
<br />were heavier. The researchers also tested media compression
<br />and found that compressing the media in the cells was not justi-
<br />fied because it was more costly and did not increase yields.
<br />In research done at the University of Kentucky, marigold plugs
<br />were grown in 512, 406, and 288 plug flats over a 16 -day pe-
<br />riod. Size did not affect seedling growth until the 13th day, but,
<br />after 16 days, seedlings grown in the 512 trays were about two -
<br />thirds the size of those grown in 288 flats. Container size also
<br />affected root branching: plants from the 288 trays had roots
<br />with fewer branches.
<br />Of greatest importance to the end user and the plug grower,
<br />however, is whether yield after transplanting is affected by
<br />container size. Researchers have yet to prove that container
<br />size affects ultimate crop yield. However, studies conducted in
<br />England in the mid -1990s showed that organically grown vege-
<br />table transplants benefited from larger cell sizes. The larger
<br />cells held more media and larger quantities of nutrients.
<br />Plug flat color has little effect on medium surface temperature,
<br />which is important when germinating seeds. Researchers in
<br />Tennessee tested black, gray, and white flats, and found no sig-
<br />nificant differences among the three colors.
<br />Grower Jay Martin in Maryland mixes Pro -Mix Lite@ and
<br />compost in proportions ranging from 12:1 to 15:1 (mix:
<br />compost), depending on the crop. For example, brassicas,
<br />which are quick to finish, are grown in a mix with less com-
<br />post; slower - growing solanaceous crops are raised in a mix
<br />with more compost.
<br />Media
<br />In organic production, plant health depends on the quality of
<br />the planting mix. Most producers opt for a soilless mix to re-
<br />duce the risk of soil -borne diseases. The mix should, for or-
<br />ganic production, contain sufficient amounts of the major nutri-
<br />ents to carry the young crop for the time it will be in the green-
<br />house.
<br />Purchasing a commercially prepared organic mix is the easiest
<br />way to get started. However, most commercial potting mixes
<br />contain synthetic ingredients and do not meet the guidelines of
<br />certified organic programs. One alternative is to arrange a spe-
<br />cial order from a commercial supplier who agrees to exclude
<br />starter fertilizers and wetting agents. Or, since freight is often
<br />prohibitive, many growers may choose to mix their own.
<br />Equipment: Seeders
<br />Mechanical seeders are necessary when planting large numbers
<br />of plugs. Seeders are quite expensive, costing from 51,000 to
<br />
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