Growing conditions
4. Place the flat, pot, or other container under lights in an area where temperatures can be maintained 18-24 degr. C. Prop up the pots (or lower the lights) to leave ~10 cm (4”) between lights and the soil surface or (later) the apex of the plants.
Lights and distance: Here at the UW-Madison, we have the luxury of growing plants in controlled environmental chambers. Specifically, our plants are grown in 11” x 22” flats placed beneath 4’, 4-bulb 32 watt T8 cool white florescent fixtures. The room temperature is held at or near 22 degr. C throughout the plant life cycle and lighting is continuous—i.e., no light/dark cycles are imposed. Flats are supported by an 11” x 22” sheet of ¼” plexiglass which is, in turn, elevated by placement of empty pots or flats to maintain a distance between plants and bulbs of ~10 cm; supports beneath the plexiglass are altered or removed as plants grow taller in order to maintain that approximate distance (figure 6).
Figure 6: Freshly planted and watered flats under lights. Lower supports are removed as plants emerge and grow; we aim to keep the apices ~4 inches from the light.
We have examined growth of FPsc under a variety of lighting and temperature conditions. Under the conditions described above, light incidence ~4” from the bulbs is ~9,000 lux (~900 ft-candles). For the sake of comparison, light intensity at the benchtop in our lab measures ~900 lux; natural light coming in the window on an overcast Wisconsin day in late September is ~3500 lux and ~37,000 lux in full 3 p.m. (late September) sunlight. In general, more light is better than less. For example, we have 55 watt T5HO fixtures in one of our growth rooms. Plants grown under those lights receive ~35,000 lux at 4” and show no ill effects. On the other hand, insufficient light can lead to problems. In particular, seedlings grown under cool white lights at a distance that yields ~5,500 lux show modest signs of etiolation (extended hypocotyls) and slight chlorosis (they are a bit paler green than seedlings grown with 9,000 lux). Ultimately, plants grown under conditions of insufficient light are spindly and weak.
A simple experiment to test whether your setup provides sufficient light intensity (and an interesting one for students to do) is to allow seeds to germinate in pots placed at variable distance from the light source(s) and measure hypocotyl length at ~ 1 week post-germination. You will notice that hypocotyl lengths will increase in proportion to the distance from the lights (intensity decreases as the square of distance). The objective is to find the distance/intensity at which no further decrease in hypocotyls length is observed as compared to plants grown under higher intensity (achieved by moving plants closer to the light). In our hands, 10,000 lux provides saturating light intensity but note that you don’t need a meter to determine if your setup is sufficient for good growth.
A couple more things on lighting are worth noting. First, there are many inexpensive lighting systems on the market—Carolina Biological Supply is a good source, as are the many horticulture sites online-- or you can rather easily build your own. Second, if you find that your lighting is not sufficiently intense, you may need to change the bulbs. Output from florescent lamps decreases over time, so new bulbs may be a quick and cheap fix. Finally, don’t bother with “grow lights”. Whereas they may be useful for propagation of certain plant varieties, we have found that they provide no discernable benefit for growth of B. rapa.
Temperature: FPsc grows well in a temperature range of 15 – 25 degr C. At temperatures towards or below the low end of that range growth is notably slower, and signs of stress and wilting are evident at or above 25 degr. C.