Introduction

The vegetable 'broccoli' is a member of the Cole Crop group and is classified as a botanical variety of the species Brassica oleracea. Readers of horticultural trade literature published in the United Kingdom should be aware that the crop in that country is referred to as 'calabrese'. The term 'broccoli' in the U.K. is often applied to a winter-hardy cauliflower, planted in the field in the late summer and maturing to a normal white curd the following spring.

General Characteristics

Morphologically, heads of cauliflower and broccoli are similar. The broccoli plant, however, generally produces a green head with longer and more slender floret-stalks than cauliflower. When the main terminal head of a broccoli plant is harvested, the axillary buds lower on the main stem are induced to develop into smaller heads, which can also be harvested.

Cultural requirements are similar to cauliflower but broccoli generally grows more rapidly. It has some frost tolerance and can be grown wherever common temperate zone vegetables are produced.

With the increased popularity of the vegetable in recent years and its high nutritional value, it is a good candidate for expanded production in southern Ontario.

Crop Scheduling

An early broccoli crop can be grown from transplants raised in plugs or modules in a greenhouse. In southern Ontario, such transplants can be planted in the field during late April, although the plants must be hardened off before being set out. Hardening-off before planting avoids a check in growth which could cause buttoning and bolting later. With satisfactory field growth, these early plantings should produce marketable heads by the middle of June. Successional plantings through the spring and summer will allow for regular harvests up to early November, or later in a mild autumn, thus providing a continuous supply to the market.

Mid to late summer is a difficult period for producing good quality broccoli because of the adverse effects of high temperatures coupled with low soil moisture. Access to irrigation is essential in these conditions.

Modern cultivars are capable of maturing to a harvestable crop within 70 days from seeding during an Ontario summer.

A reasonable guide to the earliest and latest dates for planting is as follows (in the most favorable production areas of southern Ontario):

  • Earliest date for field transplanting - 20 April
  • Latest date for direct seeding in field - 20 July
  • Latest date for field transplanting - 14 August

Note: This is only a guide, applicable to zone C as shown on the map "Climate of Ontario" in Publication 363, Vegetable Production Recommendations. Growers will need to adjust the dates depending on the climatic zone of their production location.

Crop Production Requirements

In common with other cole crops, broccoli can be established in the field by direct-seeding or by transplanting. Direct-seeding, however, holds a greater degree of risk because of possible crusting of the soil surface. This impedes seedling emergence. It is impossible to give a general recommendation on direct-seeding because many factors, such as soil type, organic matter content, and soil moisture interact to influence germination and emergence. Each grower should test this technique on a small scale and assess its practicality before large-scale seedings are undertaken.

A more reliable method of plant establishment is through the use of transplants, especially in plugs or modules raised in a greenhouse. This system gives a greater degree of control over transplant quality and timing than the raising of transplants in a field seedbed. If a field seedbed is used, successional sowings can still be made. Care should be taken to ensure a uniform population in the field seedbed. A stand of about 50 plants per running metre of row is optimum.

For module production of transplants in a greenhouse, there is a tendency to get root masses too large to be conveniently handled in mechanical transplanters without causing damage to the plants. Ideally, the soil volume for each individual module raised plant should be about 15 mL (e.g. Blackmore 200 tray).

Field spacing may be influenced by cultivation and harvesting equipment and tractor wheel positions. A spacing of 40 cm x 60 cm is a workable compromise especially using tractors with narrow wheels. The newer broccoli hybrids will perform satisfactorily as close as 30 cm x 30 cm on a bed system. Growers wishing to test the concept of a bed system of growing broccoli might consider a five-row bed with rows 30 cm apart and the tractor straddling the bed. At close spacings individual heads will be smaller, but this may not be a disadvantage since most markets still require two or three heads to be tied together in a bunch. Harvesting costs will be higher as a greater number of heads will be handled compared with wider spacings.

Insect, Disease and Weed Control

Many of the insects and diseases common to other Cole crops also attack broccoli. Control of Cabbage Looper (Trichoplusia ni) and Imported Cabbage-worm (Artogeia rapae) is particularly crucial, especially when the heads are nearing maturity. The color of these insect larvae is similar to the green of the broccoli head.

Diamondback moth larvae (Plutella xylostella) are also a serious pest of broccoli, especially close to harvest. Several factors may be contributing to this problem:

  1. Insecticide resistance by the pest: To counteract this, it is essential to alternate the chemical "families" in the spray program (i.e. synthetic pyrethroids, organophosphates and bacterials).
  2. Poor spray coverage: In order to effectively control diamondback larvae, they must be contacted by the pest control product. Installation of drop nozzles and/or application of sufficient spray volume is necessary to achieve satisfactory control.
  3. Overwintering populations: It is possible that in some areas of southern Ontario the diamondback moth is overwintering, leading to earlier infestations. Broccoli growers should monitor their early crops very closely for diamondback moth larval activity and initiate treatments at established threshold levels.

Tarnished plant bugs may also attack broccoli heads, causing symptoms similar to brown beading. Once again, growers should monitor for tarnished plant adult and nymphal activity, especially when broccoli is close to harvest.

Surface head rot can be a major constraint to broccoli production in Ontario. The condition appears to be caused by a bacterium Pseudomonas marginalis, which is soil borne and is splashed onto the developing heads through rain or irrigation. The enzymes produced by the bacteria cause tissue breakdown and the formation of putrid lesions leading to unsaleable heads. For this condition to develop, water must be present on the surface of the head. Head rot is more prevalent during wet periods or when warm humid conditions prevail during the day and are followed by cooler nights. This allows dew to collect in the small depressions on the head surface.

Chemical control measures are not available for this bacterial disease. Considerable research underway is aimed at identifying resistant cultivars and developing cultural practices which modify the crop microclimate and limit disease development.

Clubroot can be a troublesome disease with crucifer crops generally, including broccoli. The most reliable control measure is liming with finely ground lime to a pH of 7.2. Calcitic lime is preferable to dolomitic in order to attain a high level of soluble calcium (unless soils are low in magnesium).

Reference should be made to the latest editions of OMAF Publication 363, Vegetable Production Recommendations and OMAFRA Factsheets Bacterial Diseases of Cruciferous Crops (Order No. 96-046) and Fungal Diseases of Cruciferous Crops (Order No. 85-043) ) for current chemical registrations and treatment recommendations for control of insects, diseases and weeds. For Integrated Pest Management information for broccoli, consult the OMAF Publication, Integrated Pest Management for Crucifers in Ontario, Order No. 701.

Fertilizer Requirements

As with any crop, fertilizer applications for broccoli should be guided by the results of soil tests, especially for phosphate (P) and potash (K).

For nitrogen (N), broccoli should receive 130 kg N/ha, but if manure is applied, or a legume sod ploughed down, the application of N should be reduced (see appropriate tables in Publication 363). Three-quarters of the N and all of the P and K should be broadcast before planting and worked in. The remainder of the N should be side-dressed about three weeks after transplanting. If rainfall is excessive, an additional 40 kg N/ha may be side-dressed on sandy soils. For muck soils, up to 70 kg N/ha should be applied before planting and the rest split into two equal side-dressings about three weeks apart.

Head Defects

Broccoli heads are susceptible to a number of defects that are difficult to attribute to any climatic or growth aberration although some appear to be cultivar related.

Some defects can be avoided by harvesting at the correct stage so that the heads do not become overmature.

Figure 1. Head of broccoli showing surface head rot (pointer) and "starring" or "yellow-eye" condition.

Overmaturity leads to loosening of the individual spears in the head and to the premature opening of the flowers on the head surface. It is important that broccoli heads should be harvested when the bud clusters are closed and the whole head is tight and firm.

Two troublesome conditions are: (1) small green leaves or bracts growing up through the head, and (2) 'yellow-eye' or 'starring'. The latter appears to be different from the premature opening of the flower buds which may occur also. The causes of these conditions are not known but in cultivar evaluation, their persistent occurrence would disqualify a cultivar from recommendations.

Hollow stem in broccoli has frequently been linked to boron deficiency but all hollow stem is not caused by boron deficiency. Climatic and cultural conditions favoring rapid plant growth tend to stimulate the occurrence of hollow stem. Susceptibility to hollow stem differs among cultivars, although a commercial cultivar which is completely free from this disorder is not yet available.

Cultivar Choices

In recent years, the picture has changed completely from the traditional open-pollinated cultivars to hybrids. Much of the newer material originated in Japan. Many Japanese hybrids have consistently proven superior in evaluations for yield, quality, short growing season, and concentrated maturity in Ontario. Commercial seed companies in the U.S. and Holland also have promising lines with further breeding and development in progress.

Growers should check with seed company representatives annually for varieties suitable to Ontario conditions. Growers may also evaluate varieties and spacing arrangements on their own farms to determine the best and most profitable combinations.

Harvesting and Packaging

Harvesting at the correct stage and proper handling afterwards are very important with this very perishable commodity. Uniformity of maturity and concentrated harvesting are highly desirable characteristics.

Since harvesting is the single most expensive operation, it is imperative that these costs be kept to a minimum. The present trend is to harvest only the main terminal heads, usually by hand. Harvest aids are used, but complete mechanical harvesting has not been adopted, even though with optimum growing conditions and the right cultivar, a single, once-over cutting may now be economically feasible. Use of the newer hybrids has enabled growers to complete harvesting in two or, at the most, three cuts through the field.

Figure 2. Harvest aid being used in cutting broccoli.

Cultivars are certainly one factor in promoting uniformity of maturity and concentrated harvesting. However, another contributing factor is planting uniform transplants. In this respect, greenhouse grown plants raised in plugs are preferred to bare-root transplants pulled from a field seedbed. If field-grown plants are used, rigorous selection is vital so that a uniform batch of plants is set out.

The real success of broccoli growing depends on how the product is handled after harvesting. The heads usually are collected into bulk bins in the field. It is vital that the field heat be removed as quickly as possible to prevent the broccoli from wilting. Access to adequate cold storage space, between harvesting and packing, and packing and shipping, is absolutely crucial with this crop. Removal of field heat is very rapid with iced-water, ice-slurry or hydro-cooling.

Figure 3. Machine bunching and trimming of broccoli.

These methods are preferable and ideally should be used prior to forced-air cooling or standard cold storage. Loading a cold storage with "hot" broccoli will seriously delay the effective removal of field heat.

Traditionally, the market place requires that broccoli heads be bunched - two or three small heads in a bunch. This operation is now partly mechanized, but relies on hand labor to select two or three small heads for a bunch. The bunch is held together usually with elastic bands, although wire ties or plastic rings are also used. This mechanical tying process is often integrated with trimming the overall length of the bunch to about 15 to 20 cm.

Figure 4. Cartons of bunched broccoli receiving ice during packaging.

Modern cultivars, however, produce single terminal heads of excellent quality, up to at least 15 cm in diameter, the same size as two or three small heads in one bunch. Bunching of small heads was undoubtedly a logical method in the past when the older open-pollinated cultivars were grown. These did not produce such large, attractive terminal heads as the new hybrids.

It could be argued that packaging broccoli in bunches no longer conforms to present marketing and buying trends. Some markets are now known to accept single heads of broccoli, prepared and packed in a similar manner to cauliflower. Growers should explore their markets and determine precise packaging and bunching requirements.

Regardless of packing method, broccoli heads must be retained in a turgid, fresh, and attractive condition through the marketing chain right to the customer. The present practice of adding crushed or slurry ice to the cartons before they are sealed has made this possible. This single operation above all others, has given high-quality broccoli its prominent place on fresh produce shelves many hundreds of kilometres away from its production. Ontario growers who are marketing locally produced broccoli successfully are removing field heat on the farm immediately after harvesting and adding ice to the cartons during the packaging operation. Only in this way will Ontario-grown broccoli compete with imports. Growers should not embark on commercial broccoli production unless they are prepared to invest in the necessary equipment to handle the product properly.