"Greenhouse conditions" refers to an environment that is controlled and artificially created to promote plant growth, just like a greenhouse. Full definition
Could the same killer - greenhouse conditions build once again? (scientificamerican.com)
The authors acknowledged previously published research on downy mildew in basil, but noted that this study demonstrated for the first time that younger basil plants appear to be more susceptible than older plants to downy mildew under greenhouse conditions. (sciencedaily.com)
[10] Earlier still, a 200 - million year period of intermittent, widespread glaciation extending close to the equator (Snowball Earth) appears to have been ended suddenly, about 550 million years ago, by a colossal volcanic outgassing which raised the CO2 concentration of the atmosphere abruptly to 12 percent, about 350 times modern levels, causing extreme greenhouse conditions and carbonate deposition as limestone at the rate of about 1 mm per day. (newworldencyclopedia.org)