Germination & growing temperatures

Thanks to my super-early seed planting experiment, I looked up some info on germination temperatures.

For radishes, (Canadian Dept of Agriculture)
"Radish is a quick growing cool season root vegetable. The seed will germinate in 3 to 4 days with soil temperatures of 18º to 30ºC (64 to 86F) with good moisture. The minimum temperature for germination is 5ºC (41F), the optimum temperature for germination is 30ºC. The maximum temperature for germination is 35ºC (95F). Germination rates decline sharply when the soil temperature falls below 13ºC (55F). The best quality and root shape are obtained when the crop grows and matures at moderate temperatures (10 to 18ºC)(50F-64F) in intermediate to short day lengths. Radish remain in prime condition for only a few days. Roots of globe varieties tend to elongate and develop poor shape in hot weather when the tops also grow taller and larger than in cool weather. Long days induce flowering or seed stalks (bolting) and with warm weather the seed stalk may develop so rapidly that no edible root is formed. Radishes become more pungent in hot weather. Roots remain in marketable condition only a short time before becoming pithy. Growth must be continuous and rapid for good quality.

Lettuce, same source:
Head lettuce grows best at 15 to 18ºC (60F - 64F - I'll assume similar for leaf lettuce). Germination takes place at a minimum of 5ºC (41F), has an optimum range of 16 to 20ºC, and an optimum germination temperature of 20ºC (68F) (depending on the cultivar and type of lettuce). At soil temperatures over 27ºC germination is poor. Hardened seedlings are tolerant (-5ºC to -7ºC) to frost but mature plants are more sensitive to frost (-1ºC) depending on the cultivar...

Brassicas (I'm assuming the Chinese Mesclun), same source. I'm regarding these as a Cole crop, although that may not be accurate. The brassica family are well adapted to cool season production. These plants are quite cold resistant. Young hardened cabbage plants can withstand -10 for a short time, older plants are less hardy. The growth rate of cabbage stops at 0°C and is quickest at 15°C to 20°C. Above 25°C growth stops. ... The minimum temperature for seed germination is 5°C with an optimum germination temperature of 27°C, an optimum range of 7 to 27°C and a maximum germination temperature of 37°C. Cauliflower and broccoli will not stand temperatures as high or low as cabbage....

And finally, carrots, same source:
Optimum growing temperatures for these crops are 15° to 20°C with a minimum of 5°C and a maximum of 24°C. The minimum temperature for germination is 2°C (35.6F); with an optimum range of 10° to 25°C. The optimum germination temperature for... carrots is 25°C. The maximum temperature for germination is... 35°C... These crops therefore favor cool season conditions. Low and high temperatures reduce seed germination. Both carrot & parsnip foliage are hurt by frosts (1.5°C) but this does not usually affect the roots. When there are freezes for over 24 hours, the crowns can be injured and these carrots will not keep well. Adequate moisture is necessary for good yields and quality....When (parsnip) seedlings with roots 6 mm in diameter or larger are subjected to cold temperatures (below 10°C) for a period of time flower initiation takes place. For early seeded carrots of susceptible varieties, bolting may occur.

So, it looks like the current temps or a bit warmer will be OK. I found an old translucent shower curtain, cut it to fit, and covered each barrel. That should let in infrared light and warm the barrels a bitg during the day. I'll look for a thermometer.