The CRC Handbook of Flowering suggests 25-32C is required. An article by Summerfield et al., suggests 24-26C.

Temperature was not a variable studied in our two studies. However, we did not observe any flowering in the first study conducted at 24C day/18.3C night temperatures, while we did observe flowering with our second study conducted at 26.1C/22.2C. Informal surveys of other research institutions indicate that 26C daytime is a minimum required for flowering, with some using as high as 30C. These combined reports confirm the recommendation in the CRC Handbook.

No, but it would hasten flowering.

According to the CRC Handbook, cultivars differ in the timing of their photoperiod sensitivity, and in their optimum photoperiod. Nine to ten hours was suggested as an optimum during the period of sensitivity. It should be noted that Summerfield et al., (1991), suggests that japonica rice is less sensitive to photoperiod than indica rice. We chose not to apply short-day treatments because of the labor scheduling it would require. In a greenhouse, short days are applied by pulling black-out fabric around the plants to ensure dark conditions. In our experiment, flower panicles emerged on day 93.

The greatest plant height and tiller number were achieved with the Profile “Greens” calcined clay granules, when fertilized twice weekly. These granules are made for the golf course industry to improve drainage.

This treatment also resulted in a “no chlorosis” ranking, though several others achieved this color ranking. Other root media that resulted in vigorous growth were Pro-Mix ‘BX’/Profile and Pro-Mix/Turface, each at a 1:1 ratio by volume, though these media did result in some chlorosis.

(Editor's note: Since the study's completion, we have used Fafard #2 and Fafard 1-P soilless mixes effectively at a similar 1:1 ratio with Profile.)

Plants failed to thrive in the clay loam mineral soil by all growth measures. This is a significant outcome, as many research institutions grow rice in this root media to avoid chlorosis. Of course, mineral soil varies by what can be dug locally, but there is no reason to believe this soil is not conducive to growth, being pH balanced to 6.2 and capable of growing other grass crops in our university greenhouses. We suggest that changing this production component alone may optimize growth of greenhouse rice for many research projects.

An observation that will come as no surprise to rice researchers is the poor growth observed in a peat-based soil-less mix. Severe chlorosis developed, which would have required multiple micronutrient applications to correct. Without these corrective applications, chlorosis worsened, resulting in short plants and fewer tillers than other treatments.

Germination was suitable in all the root media, but slower in mineral soil and Profile. At day 10 of the study, germination was 42-96% in the other media, but just 17% in the mineral soil and 21% in the Profile. This low percentage in the Profile medium did not result in less vigorous plants as the study continued, however. Profile calcined clay granules do seem like a radical departure from the media our researchers are use to; we often recommend to start with a 1:1 mix of Pro-Mix or Fafard Mix and Profile to get use to growing in it and then move to the Profile medium to achieve optimum growth potential.

For experiments requiring mineral soil component or for researchers reluctant to change methodology, we recommend augmenting mineral soils with Turface to improve drainage.

Note that both Profile and Turface are clay products so have a high cation exchange capacity: They are not inert like some soil mix components such as perlite.

Under the conditions of this study, 9-cm (4-inch) diameter square pots resulted in the greatest seed yield and panicle number as compared to 7-cm (3-inch) and 12.5-cm (5-inch) diameter pots.

Throughout the experiment the larger pots, 12.5-cm (5-inch), seemed sub-optimal. This seems counter-intuitive, as the larger pots provide more soil volume. Our best explanation is that the 4-inch pots coupled with these sub-irrigation trays had a more ideal balance of air and water in the root environment due to the pot height, volume and shape. In other words, if the 3-cm deep sub-irrigation trays that we used were either deeper or shallower, another pot size may have proven most optimum. So it is important to remember that the components of a plant growth system are not independent of each other.

The 7-cm pots (3-inch) square pots yielded seed despite a very small soil volume. We believe this is the first report of successful growth of rice in such a small container, and that this bears closer examination by research institutions involved in growing large numbers of plants, either for breeding or high-throughput screening. Grown in less-dense conditions than were provided in this study, this pot size may prove beneficial. The disadvantage would be the need for careful monitoring of irrigation to avoid water stress.

Though data was not collected, our observations were that pot size did not effect the number of days required for flowering.

(Editor's note: Though not included in this study, 15-cm [6-inch pots] are more stable and forgiving of water stress events if enough space is available for these larger pots. Planting multiple seeds per pot may reduce or eliminate this advantage, however. A 1:1 mix may be required as well, as the pot is too tall to wick up water with Profile alone as root medium. )

In this study, we compared drip irrigation and constant sub-irrigation by keeping a 3-cm tray filled with 1-3 cm solution. Constant sub-irrigation resulted in best growth.

Drip irrigation was a suitable watering method but did not result in plants as vigorous as those using the sub-irrigation method. Plant height was greater in sub-irrigated plants over drip-irrigated plants in all but three root media and fertilizer frequency treatments. Tiller number was greater in sub-irrigated plants over drip-irrigated plants in all but two root media and fertilizer frequency treatments. Likewise, chlorosis occurred less in sub-irrigated plants over drip-irrigated plants in all but two treatments. The only plants that grew better with drip-irrigation over their sub-irrigated counterparts were grown in Pro-Mix.

One likely explanation for the less vigorous growth using drip irrigation was that the twice daily irrigation with clear water leached away nutrients. A growing system could most likely be devised utilizing drip irrigation for rice if the problems of wasted water and nitrogen run-off could be addressed, perhaps with one or a combination of slow-release fertilizer, water recirculation or low-volume application. Our findings go against the conventional wisdom that rice requires constant sub-irrigation—simulating paddy culture—to survive.

Because of the poor suitability of the drip-irrigation used in this study, results discussed in this and other reports of “Optimizing Greenhouse Rice Production” for root media and fertilizer application frequency will be solely from the sub-irrigated plants. Plant responses to root media and fertilizer frequency were relatively similar between the two irrigation types, only less vigorous for the drip irrigation group except where noted.

Over the course of two studies, we compared a standard greenhouse fertilizer solution applied once per week, twice per week, and constantly by keeping a 3-cm tray filled with 1-3 cm of the solution. We concluded that twice weekly fertilization resulted in more vigorous plants than once weekly or constant fertilization.

In the studies, we compared results using the root medium we had determined the best for rice production, Profile “Greens” calcined clay granules. Our results may not apply to other root media. Likewise, results may not apply if using a different fertilizer formulation or strength. See our Materials and Methods report for details of the 15-5-15 fertilizer used.

In the first study, plants fertilized twice weekly had greater height and tiller count than once weekly, and less chlorosis when fertilized twice weekly.

In the second study, the twice weekly fertilization frequency resulted in greater seed weight over the constant fertilization. Panicle count was higher under the twice weekly regime for two of the three pot sizes compared. Seed yield was highest in the 4-inch square pot for the set fertilized twice weekly, and highest for the 5-inch azalea pot for the constantly fertilized set.

Plants in both studies were more responsive to fertilizer in the first half of their life cycle, particularly in maintaining leaf color during rapid phase of vegetative growth. This suggests that plants could be constantly fertilized for this early cycle (30-45 days), then fertilizer reduced by applying twice weekly until maturity. Further study is needed to confirm this possibility.

Seed yields of plants fertilized twice weekly were at least double of corresponding plants fertilized constantly. It is difficult to determine if the constantly fertilized plants had less yield due to a toxicity of some element or excess salt damaging roots, or if it was the result of lower light due to the lush tiller growth. (Plant spacing was not controlled in this study. See: What is best plant spacing?). Whatever the explanation, the constant fertilization treatment does appear sub-optimal.

(Editor's note: Subsequent production of rice using constant fertilization has verified that plants become damaged after about 45 days. The damage did not correlate with high salts in soil as measured by electroconductivity. The damage was less severe using a root medium of 1:1 soilless mix:Profile than for Profile alone.)

We did not formally compare differing plant spacing, but can recommend a density of no more than 40 plants per square meter (about 4 plants per square foot).

If researchers use the same 10x50-cm sub-irrigation tray as we did, this is 5-6 plants per tray. Proper spacing will depend somewhat on pot size and vigor of plants, the goal being to have suitable light for flowering and suitable air circulation to prevent diseases. Another advantage of properly spaced plants is to make pest control easier using water sprays or applications of pesticides.

In our study comparing pot sizes, pots were packed into the sub-irrigation tray tightly, so that they would not tip over when the tray was drained and fresh water added to reduce algae. This configuration was a flaw in the study, as it meant that plant density was not controlled: the density varied from 6 pots per tray to 21 pots per tray from largest to smallest pots. This did not appear to reduce vigorous vegetative growth but did contribute to flower panicle and seed yield differences. However, we observed growth and flowering of some extra plants that were planted but not required for the study that were given more space. The photographs suggest that yield was improved.

Our recommendation of 9-cm (4-inch) square pots containing 'Profile Greens' calcined-clay granules, sub-irrigated constantly in 2-3 cm clear water and fertilizer twice weekly, produced excellent plants without need supplemental micronutrient treatments.

Leaf yellowing observed early in the production cycle of greenhouse rice is usually iron deficiency. Soilless media should be avoided for rice, though augmenting with mineral soil or calcined clay granules reduces this chlorosis.

Supplemental applications of iron are achieved with chelated iron, iron sulfate, or micronutrient fertilizer. In our greenhouses, we use a solution of Sprint330 at a rate of 0.3g/liter applied to the soil. One or two follow up applications are usually required.

No, in our study plants failed to thrive when grown in our clay loam mineral soil by all measures.

Augmenting the soil with Pro-mix, Profile or Turface at a 1:1 by volume ratio improved growth and tiller count, but did not produce as vigorous plants as some of the treatments that did not contain the field soil.

This is a significant outcome, as many research institutions grow rice in this root media to avoid chlorosis. Of course, the mineral soil varies by what is available locally, but there is no reason to believe the soil we used is not as conducive to growth as others, being pH balanced to 6.2 and capable of growing other grass crops in our university greenhouses. Changing this production component alone may optimize growth of greenhouse rice for many research projects.

No, we grew suitable plants with drip irrigation.

Only if water testing indicates you have very poor water quality.

Though we consider our clear water to be of poor quality due to high alkalinity, we’ve seen no visible differences between Arabidopsis plants irrigated with reverse-osmosis purified water versus our tap water. Reverse osmosis or other purified water is necessary in laboratory culture or hydroponics, but usually not in crop production in soilless media. We also use it for mixing pesticides, to make them more effective.

Plants growing in calcined clay granules, both Profile Greens and Turface MVP, appeared cleaner of debris than other media.

Drain trays twice per week.

Do not let fertilizer water stand in trays. If practical, change or disinfect trays if algae growth begins, and rinse off pots where algae clings. Interestingly, algae growth declines later in the production cycle, perhaps the result of more frequent need to refill trays. We have informally examined the use of GreenClean algaecide granules in the standing water of the trays. It did not reduce algae already present, but did not harm plants at label-recommended rates. More testing needs to be done with the granules applied before algae is present.

(Editor's note: A controlled study at another university indicated Zerotol disinfectant applied to water in sub-irrigation tray controlled algae without damage to plants. Data not provided.)

For our study with O. sativa japonica 'Nipponbare', about 93 days from sowing.

For our study with O. sativa japonica 'Nipponbare', about 112 days from sowing.

An integrated approach of good cultural practices, scouting, hosing off pests, removing infested leaves, introduction of predatory mites or other beneficial insects, and pesticide applications if needed.