I have grown these plants in the summertime with great success, but I'm having trouble growing them well this winter. Germination is not a problem (I soak seeds in 2000 ppm gibberellic acid overnight and get sprouting within a week), but the seedlings placed in the greenhouse quickly become leggy and further development is very slow and stunted. Because I use the plants I grow to feed insects (that is part of the research), I am not able to use pesticides and many of my plants have become damaged by thrips if they do not grow fast enough.

The bay receives about 8hrs of sun now in the winter and the greenhouse thermostat is set at 30 C /20 C (day:night), though the bay does not always hold those temperatures. I don't have supplemental lighting, with the exception of a couple of cool fluorescent fixtures.

Anyone have tips for successfully growing weedy annuals in the greenhouse in the winter?

Thanks

It sounds like your greenhouse may be a bit warm, especially for the amount of light your plants are getting. Try gradually cooling it down by about 10° C. Check the humidity and, if there isn't one, install a fan to freshen and circulate the air. Reduce the amount of fertilizer your seedlings are getting. Remove unwanted insects by spraying them with a stream of water (larger plants can be washed down in a gentle, lukewarm shower).