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Grower Solutions Magazine
Lefroy Valley 

Dec 2002

Broccoli can be harvested all year round in Southern growing regions, by careful management of the specific variety that is being grown at any particular time of the year.  All varieties have different vernalisation requirements, where vernalisation is the amount of time the plant needs to be exposed to cool temperatures, to begin head formation.

Broccoli plant development

The development of a broccoli plant can be divided into three distinct phases; a juvenile phase, a head induction phase and a head growth phase (Grevsen, 1998). In the juvenile phase, vernalisation is not able to occur, as the plant has in built mechanisms to stop it producing a head, while it is too small. Once the plant reaches a critical size (as expressed by a minimum stem diameter) and the temperatures are cool enough to meet the vernalisation requirement of the variety being grown, cold can be accepted as a signal to begin head formation (Miller, 1985). It is this difference in vernalisation requirement that enables broccoli to be grown when the temperature is 5C or 35C, but obviously the variety that can be grown under those environmental conditions is not the same.

Vernalisation differences in winter grown & summer grown broccoli

Varieties that are harvested in winter, have relatively high vernalisation requirements, meaning that the plant needs to be exposed to many days of cool temperatures, to begin head formation. An example of such a variety is Marathon.

Alternatively, summer harvest varieties have much lower vernalisation requirements, meaning that exposure to cool temperatures can cause the plant to produce a head early in its growth stage, once it has gone beyond the juvenile phase. For early varieties, which can mature approximately 100 days from sowing in mid summer, the juvenile phase can end as little as five weeks from sowing. After that stage in its development, cold temperatures can lead to premature head formation.


With heat tolerant varieties like Viper, a variety that is recommended for harvest in the hottest part of summer, several factors play a critical role in determining causes of premature head formation. Studies have shown that the percentage of premature heads will be increased by the use of larger transplants (Bagget and Mack, 1970), supporting the theory that once a critical size is reached, head formation can occur, even if the plant is too small to produce a marketable head. A critical age of transplant, also related to size exists, as three week old seedlings did not button under temperatures of 4C, but all five week old plants buttoned prematurely under those conditions (Bagget and Mack, 1970).

The cooler the temperature is, the earlier head induction and buttoning can take place, once the plant reaches a critical size. (Wurr,,1995). The table shows the plants begin head induction sooner, if the temperature is cooler. Alternatively, if the temperature is relatively warm, the plant begins head induction later. Table: Temperatures required to cause head formation as broccoli plant dry weight increases (Wurr,,1995).

Once a broccoli plant has moved into the head initiation phase, any extra stress, be it related to moisture, fertiliser, transplant shock pests and disease, can all cause the percentage of plants that begin to button prematurely to increase further.

Experiences of the summer of 2001/02

During the early summer of 2001/02, Southern Australia experienced some very low temperatures. In the Werribee area of Victoria, this was extremely apparent during November, where nights were 4C cooler on average than the previous November and December, where days were 5C cooler than in December 2000.

Some growers who grew summer-harvest broccoli during that cool period, found a percentage of their crop buttoned prematurely. The previous studies on premature buttoning in broccoli can clearly explain why this occurred.


From the literature, we now that fast maturing, summer harvest varieties have relatively low vernalisation requirements, compared to a winter harvest variety like Marathon. From experience over the past summer, the critical stage after which cold stresses can cause premature head formation in fast maturing varities, such as Viper, appears to be at approx. the 5-6 true leaf stage. It is very easy to see at what stage the plant goes from its normal vegetative phase to its reproductive phase, where it begins forming a head, by looking at the leaves. In the vegetative phase, the leaves are pointy (Fig. 1) and have a ruffled edge, while the leaves in the reproductive phase tend to be rounder and have a flat edge.

Performance of Viper in Southern Australia in 2001/2

In previous summers, when Viper was first trialed, the relatively warmer weather in those years meant that the vernalisation requirement of Viper was not met at an early growth stage, so the plant developed normally.

In previous summers, when Viper was first trialed, the normal warmer weather in those years meant that the vernalisation requirement of Viper was not met at an early growth stage, so the plant developed normally. Despite this, Viper was only recommended for growing in the hottest part of the Greenbelt slot. For instance if you have been harvesting Greenbelt between November and May you would only harvest Viper from January to mid April. This past season, some growers of Viper experienced a minor problem with premature buttoning, while a small number of growers had a significant percentage of their crops button early. In some cases, adjacent fields, transplanted on the same day with plants from two different grower nurseries, saw extreme variations in buttoning, with one field having 2% early buttoning, the other 40%.

There are several factors that could explain why neighbouring fields, transplanted on the same day, with transplants from two

sources had such a dramatic difference in the early buttoning percentage. The single largest factor , which we are unable to control, is the weather. The cold start to summer predisposed the plants to head induction signals. Regardless of any cultural practices, if it is cold enough in early summer, varieties such as Viper may button prematurely. From the literature, the size and age of the transplants appear to have a significant bearing on the buttoning percentage. Younger and smaller transplants are less likely to button early. Once out in the field, minimisation of any post - transplant stress is also important in reducing the percentage of premature buttoning.

In Summary to minimise premature buttoning in fast maturing summer harvest broccoli:

Climate - we are unable to control, but cold weather can cause premature head formation. Transplants - use smaller, younger plants where possible.
Field Conditions - minimise any sort of transplant and growing stress - moisture, fertiliser, pests, diseases.


Baggot, J.R., and Mack, H.J., (1970). Premature Heading of Broccoli Transplants as Affected by Transplant Size. Journal of the American Society for Horticultural Science, 95, pp. 403 - 407.

Grevsen, K. (1998). Effects of Temperature on Head Growth of Broccoli (Brassica oleracea L. var. italica): Parameter Estimates for a Predictive Model. Journal of Horticultural Science and Biotechnology, 73, pp. 235-244. Miller, C.H., Konsler,T.R., and Lamont,W.J. (1985). Cold Stress Influence on Premature Heading in Broccoli. HortScience, 20, pp. 193-195.

Wurr, D.C.E, Fellows, J.R., Phelps, K., and Reader, R.J. (1995). Vernalisation in Calabrese (Brassica oleracea L.Var. italica) - A Model for Apex Development. Journal of Experimental Botany, 46, pp. 1487-1496.

Bruno Tigani

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