Auckland Botanic gardens

Samantha van Ryn[1], Ella Rawcliffe, Jodie McDonnell

 

Introduction

 

Wild maize (Zea mays) has been cultivated since around 7000BC, when the first crop was found in Mexico. It became a staple crop in North America when it was introduced in 800BC, and is now the most commonly grown crop, used as biofuel, animal feed, and in almost all processed foods, although less than 2% is directly consumed by humans (WRI, 2022). There are more than 500 different by-products of maize. Maize was introduced to Europe as sweet corn and meal and is now the second most important food crop in the world (G.o.C., 2020). Popcorn (select cultivars of Zea mays) can be distinguished from traditional maize, sweetcorn (Zea mays var. saccharata, containing more sugar than starch) and flour corn (Zea mays var. amylacea, with soft endosperm good for grinding) as it contains starch granules which explode upon heating.

Auckland’s subtropical climate makes growing some Zea mays cultivars difficult due to the high potential for pests and diseases to infect plants. Zea mays is often grown outdoors during summer when the climate is humid and hot. The most common problems are aphids, scale, fungal diseases while growing, and weevils once harvested (Lucas, 2005).

Auckland Botanic Gardens (ABG) uses plant trials to test and identify plants that do well in Auckland conditions with desirable characteristics including little or no pests or diseases, long floral displays, productive harvest, uniform habits, and minimal maintenance requirements. This allows ABG to make recommendations of plants suited to the Auckland climate and ensures high performing plants are displayed in the gardens. The aim of this trial was to find top performing popcorn selections that produce a healthy and edible crop that stays pest and disease free, whilst still creating an attractive display. 

Methods

Five cultivars of commercially available popcorn from Egmont Seeds, Kōanga Institute and Kings Seeds were selected for the trial, based on their advertised appearance, disease resistance and kernel quality (Table 1). Popcorn seeds were sown in the Botanic Gardens nursery in September 2024, grown on, and planted out in October 2024 in the trials garden at ABG. 25 of each cultivar were planted in blocks. Each cultivar was planted in three rows running north to south and exposed to the prevailing wind. Plants were spaced 20 cm apart. Prior to planting, trial beds were prepared with compost and mulch. Plants were watered during the first two weeks after planting, and snail bait was used once during this time to ensure they had a fair start to life. The only subsequent maintenance was occasional watering, when the soil became particularly dry (no more than once per fortnight).

 

Table 1  General information and source of five Zea mays cultivars.

Zea mays cultivar	F1/Heirloom	Kernel Colour	Source
Z. ‘Ruby Red’ (Fig. 1)	F1 hybrid	Red	Egmont Seeds
Z. ‘Strawberry’ (Fig. 2)	Heirloom	Red	Kōanga Institute
Z. ‘Glass Gem’ (Fig. 3)	Heirloom	Rainbow	Kings Seeds
Z. ‘Mini Black’ (Fig. 4)	Heirloom	Black	Kōanga Institute
Z. ‘Mushroom’ (Fig. 5)	F1 hybrid	Yellow	Egmont Seeds

 

 

 

 

 

 

 

 

 

At maturity, the average height and width (cm) of plants was measured prior to harvest. Pest and disease prevalence was recorded (as percentage affected) upon harvest. An evaluation by a group of staff was conducted in February to assess plants overall and rate them according to ABG star performer criteria (1 = poor performer to 10 = excellent performer). Cultivars that scored 8 or more were considered top performers and are recommended for Auckland conditions based on the results of these trials. The overall rating took into consideration the quality and yield of cobs, absence of pests and diseases, plant habit, and vigour. All cobs were harvested and two of each cultivar were subjected to a qualitative taste test by staff. Cobs were left to dry for two months on the plant, then laid out to dry out for two months in a dry environment.

 

Results

Upon maturity, the tallest and most vigorous cultivars were Z. ‘Mini Black’, ‘Ruby Red’ and ‘Glass Gem’ (Table 2). All cultivars were afflicted by aphids, with ‘Mushroom’ the worst afflicted. Some cultivars were affected by ‘tassel ear’[1] which is likely to be a result of soil compaction and planting layout. We recommend planting corn as densely as possible, and to plant individual cultivars separate from each other to avoid cross-pollination and potential reduction in cob quality.

All cultivars had ripened cobs by mid-January 2025 but were left to dry on the plant. Z. ‘Strawberry’ and ‘Mushroom’ were harvested in mid-February, and the three remaining cultivars were harvested at the end of February. All corn was left to dry in trays at room temperature, and after popping, was subject to a taste test in mid-May. Upon harvest, Z. ‘Mini Black’ and ‘Glass Gem’ were the most prolific producers, while ‘Mushroom’ and ‘Ruby Red’ had the largest cobs (Table 3). Z. ‘Mushroom’ and ‘Glass Gem’ had 75% of kernels fertilised, while the remaining three cultivars had all kernels fertilised. While drying inside, it was noted that all cultivars except ‘Mini Black’ were damaged by a mining insect (likely a caterpillar) (Fig. 6).

 

Table 2  Height and width (cm), pest damage, and overall rating of five Zea mays cultivars. Star performers are indicated with an asterisk (*).