The growth and flowering of all bulbous and tuberous flowers in greenhouses is very much dependent on the temperature not only when they are growing, but much more so when they are dormant.
For a long time, for unknown reasons, freesia corms germinated well in some years, with normal growth and flowering, and did not germinate at all in others, even though normal life processes were taking place in the ground. In 1935, research began to investigate why this was happening.
Dutch scientists A. Hartsema and Z. Luyten first pointed out that such phenomena did not occur in southern France, which is much warmer than the Netherlands. The researchers stored the freesia tubers for 10 weeks at different temperatures: 9, 20, 23, 25 and 28 °C. It turned out that only the tubers stored at 9 degrees Celsius showed poor germination. The best germination was observed when the tubers were kept in warm conditions. They later found that the optimum storage temperature was even higher, 31 °C. The tubers should be kept there for 10 weeks. Other researchers have shown that tubers should be kept at 28-31 °C for an even longer period of 3-4 months before they germinate well and flower profusely.
Many experts believe that warm-kept tubers should be kept at 13 °C for 3-4 weeks before planting. Some authors (A. Hartsema, K. Kukulčanka) claim that such tubers grow well and flower early, while others (B. M. Mansour, H. Jensen) have not confirmed this. It is reasonable to say that freesias with corms stored at 13 degrees Celsius produce long inflorescences, which is very important. Tubers kept at this temperature for more than 4 weeks do not germinate until 3-4 months later.
Tubers kept at high temperatures for more than 4 months suffer a significant reduction in size due to the depletion of nutrients due to intense biochemical processes. But what about when tubers need to be dormant for a long time?
Biological processes in corms
After the plants have flowered, new tubers start to grow vigorously and children form in the base of the former leaves. If the excavated tubers are kept at 20 degrees Celsius, the children grow during the dormant period, taking nutrients from the mother tuber. Van de Nees (A.G.A. Van de Nees) stored freesia tubers at 13 °C for 9 months in 1957. During this time, new tubers grew, but their weight was only 60 per cent of that of the mother tubers. Thus, the nutrients from the mother tubers were transferred to the daughter tubers, but a large part of them was used up in bioenergetic processes. The present author also found that biological processes do not occur in tubers when they are stored at 1-2 °C. They can survive for 9-11 months under these conditions, but the tubers still need to be kept at 28-31 °C for 3-4 months before planting.
Many researchers have studied the effect of different temperatures on freesia tubers and dormant tubers. In addition to these, the Dutchman C. J. Kragtwijk (1961), the Danes A. Klougart and E. Jorgesen (1962), the Japanese S. Abe, J. Kawata, A. Utada (1964), K. K. K. K. K. (1964) and C. K. K. K. (1964). Kosugi, A. Sumitoma (1955).
The physiological effect of temperature on tubers during dormancy was substantiated by the Danish scientist F. Renstrom in 1965. He found that freesias grow normally only when their tubers contain sufficient auxin-like growth stimulants. These stimulants are synthesised in the tubers at high temperatures during dormancy. No auxins were found in tubers stored at 5 °C and few were found at 10 °C. This amount was only sufficient to produce roots and new tubers.
The duration of cultivation, the yield and thus the economic viability of the plants depend on the correct temperature regime during the dormant period.
It is therefore important for freesia growers to be aware that only tubers that have been stored at 28-31 °C for 3-4 months (relative humidity of 65-80 %) can be planted. When the tubers need to remain dormant for a longer period of time, they should be stored in a refrigerator (1-5 °C). They should then still be kept at a high temperature (28-31 °C) for 3-4 months before planting.