Spotlight on: Heat-tolerant varieties

As heatwave events become more frequent across Australia’s growing regions, the ability of crops to withstand high temperatures is becoming increasingly important. Heat stress can impact plant growth, fruit set, quality and overall yield, making heat tolerance a key focus in modern vegetable breeding. Through careful selection and extensive field trials, plant breeders work to develop varieties that maintain productivity and quality even when temperatures rise.

What is the impact of high or heatwave temperatures on different crops?

Heatwave conditions can have a range of impacts on various crops, from reducing photosynthesis in plants, to burning leaves, and damaging pollen as well as the cells of plants. Of course, there are other impacts to ecosystems, including reduced pollination from insects and bees, reduced water quality and negative impacts to soil microbiology.

During Australia’s unprecedented heatwave in late January 2026, air temperatures reached 50°C in inland South Australia. Western Australian mango growers experienced losses of up 40 per cent as fruit ‘cooked’ in the field, and farmers across Victoria contended with bushfires that persisted throughout much of January. In Queensland, the extreme heat came with increased rain, tropical cyclones and flooding.

Heatwaves tend to increase issues such as transplant loss, sunburn and tip burn, as well as increased pest and disease stress, alongside broader impacts on market supply.

Rijk Zwaan local representative in South Australia, Steve Natsias says increasing irrigation is one of the key techniques growers use to manage heat stress. “High levels of dissolved salts in many water sources across Australia during summer can add to the existing stress on plants. Of course, it also adds to the growers’ expenses, and ecological costs,” says Steve.

“Transplant loss during these kinds of conditions is a major risk, and even days’ delay to a growing schedule will mean the possibility of not fulfilling market contracts, so finding varieties that stand up well in these conditions is critical for Australian growers.”

Speaking with RZ representatives across the country, here are just a handful of the varieties that were put to the test during this year's tough climatic conditions and came out with flying colours.

How did different RZ varieties stand up to heat stress this summer?

Working with tomato growers in South Australia, Steve observed Adventure RZ and Endeavour RZ as two medium-sized truss varieties that stood up exceptionally to consecutive days of extreme heat.

“Our Adventure and Endeavour tomatoes deliver strong harvest outcomes both as truss and loose harvest,” said Steve.

“They both showed good vigour under very tough conditions, sustained fruit set and produced firm fruit in both protected and mid- to high-tech growing systems.”

In Victoria, Client Manager and Crop Specialist for lettuce, Rob Philip was pleased to see a relatively new variety of easy-cut lettuce called Police RZ doing exceptionally well under the conditions.

“The firmness and the shelf life of this easy-harvest iceberg have stood out this summer,” said Rob.

“It’s performing well in WA, VIC, and QLD where it is growing in a range of conditions. It offers a slightly raised stem and upright outer leaves with an oval shape, which increases field density, leading to higher yields per hectare and an efficient harvest, whether done by machine or by hand.

“The smaller cutting surface allows for quicker removal of the upright outer leaves, leaving the marketable head intact, and it’s suitable for a wide variety of growers in many parts of Australia,” said Rob.

In the north of Victoria, Client Manager Cameron Cornish has been paying particular attention to how the state’s melon harvest has held up in extreme heatwaves followed by floods.

“Amberetta is a bright golden-skinned honeydew melon that has stood up to some extreme conditions in the field this summer,” said Cameron.

“It’s really everything you could want in a variety— excellent flavour with consistently high Brix, outstanding flesh colour, vigorous plant health that withstood many consecutive hot days, and just the right sizing for existing markets.

Over in Queensland, Client Manager, Eva Sarosi has turned her attention to celery that can take everything the climate throws at it.

“The celery variety Girisha RZ has produced excellent results through heat and high rainfall this past summer,” said Eva.

“Girisha RZ is a medium-dark-green blanched celery with upright, uniform sticks and consistent knuckle height. The variety shows strong tolerance to leaf diseases, including Septoria and Cercospora.”

Developing varieties that can perform reliably under heat stress is an increasingly important focus for Australian growers. Through ongoing trials and feedback from growers across Australia, Rijk Zwaan continues to evaluate how varieties perform across the country’s diverse climates — helping ensure growers have genetics that can stand up to increasingly challenging conditions.

As Managing Director Australia, Peter Smith explains: “We invest 30% of our annual turnover back into R&D. This enables us to develop varieties with resilience: less susceptible to plant diseases, less dependent on chemical crop protection agents, and better able to cope with extreme heat or drought.”

Contact your local client manager to discuss your variety requirements.