Zucchini

Start from seed: October-November

Optimum soil temperature for germination: 21-35°C

Plant seedlings: November-December

Plant spacing: 50-60 cm

Days to maturity: 45-55 days

Introduction

Zucchini, Cucurbita pepo, also known as courgette, belongs to the Cucurbitaceae family, which includes cucumbers, melons, and pumpkins.

Zucchini originated in Central and South America, where it has been cultivated for thousands of years. It was brought to Europe by explorers in the 16th century and became particularly popular in Italian cuisine, which led to many of the varieties we know today. The name “zucchini” comes from the Italian word “zucchino,” meaning small squash. This explains why they are often referred to as squash.

The plants are large, sprawling somewhat prickly bushes with large, broad leaves that often have a distinctive mottled pattern. They produce bright yellow flowers that are also edible. Botanically, the zucchini fruit is classified as a berry, specifically a “pepo,” which is a type of berry with a thick rind and many seeds.

Site and soil

Zucchini can be easily grown outdoors in Canberra with proper variety selection and timing. Being a fruiting plant, zucchini requires at least 6 hours of full sun per day. Choose a warm, sunny, sheltered location protected from strong winds.

Prior to planting, prepare the soil by adding compost and complete organic fertiliser. Zucchini prefers fertile, well-drained soil with a pH between 5.8-6.8. Raised beds can help improve drainage in heavy soils.

If growing in pots, it is very important to use a high-quality potting mix boosted with complete organic fertiliser. Choose a container at least 40-50 cm in diameter and depth to accommodate the plant’s extensive root system.

For details on preparing the soil and pots, including the use of lime, dolomite, gypsum and fertilisers please refer to the COGS guide to garden bed preparation.

Planting Seeds and Seedlings

Zucchini can be started from seed or purchased as seedlings. If starting from seed, sow direct into the soil outdoors after all danger of frost has passed (usually mid-October to early November in Canberra). Alternatively, seedlings can be raised indoors for planting out 3 to 4 weeks later. This method has advantages because the seedlings are protected from frost then planted out when advanced leading to earlier fruiting. If there is danger of frost or you want to start them early outdoors, plants can be protected by covering with caterpillar tunnels. The optimum soil temperature for germination is 21-35°C, with little to no germination occurring below 15°C. Seeds should germinate within 10 days under optimal conditions.

When direct seeding, sow 2-3 seeds per position and thin to the strongest seedling once they develop their first true leaves. For transplants, harden off seedlings by gradually exposing them to outdoor conditions over a period of 7-10 days before planting out.

Space plants 50-60 cm apart to allow for adequate air circulation and the sprawling growth habit. After planting, water thoroughly with a seaweed solution to reduce transplant shock and promote root development.

Applying organic mulch such as straw, lucerne, or sugar cane mulch will help conserve moisture, suppress weeds, and regulate soil temperature.

Cultivation

Zucchini plants are frost-sensitive, so seedlings should only be planted out after the danger of frost has passed and the soil has warmed. In Canberra, this is typically from mid-November onwards. If a late frost is forecast, protect young plants with cloches, frost cloth, or upturned buckets.

Consistent watering is essential for zucchini production. Aim to provide 2-3 cm of water per week, applied directly to the soil rather than overhead to prevent foliar diseases. Water deeply and less frequently rather in preference to frequent, shallow watering to encourage deeper root growth.

Plants may need a side dressing of complete organic fertiliser when the first fruits begin to form. A monthly foliar spray with seaweed solution; a mix of worm juice, seaweed and compost tea; or trace elements will help boost growth and resilience to pests and diseases.

Harvesting

Harvest zucchini regularly, ideally every second day, once plants begin producing. Younger, smaller zucchini (15-20 cm long for elongated varieties, 5-7.5 cm in diameter for round types) have the best flavour and texture. Regular harvesting encourages continued production.

To harvest, cut or gently twist off fruits when they have reached the desired size. Use a sharp knife or secateurs to cut the stem about 2-3 cm above the fruit, being careful not to damage the plant. Handle fruits with care to avoid scratching or bruising.

Some varieties, like ‘Striato d’Italia’, grow long before increasing in girth, allowing them to be harvested at a good size before becoming too large. Others, like ‘Rond di Nice’, are round and should be harvested when they reach 5-7.5 cm in diameter.

The flowers are edible and are a gourmet delicacy. Harvest male blossoms (with thin stems) or female blossoms (with thick stems and a round swollen immature fruit at the base) in mid to late morning when fully open. If you also want to harvest fruit, only pick male flowers, leaving a few for pollination. If fruit production is excessive some female flowers can be removed.

Problems

There are numerous pests and diseases that can affect zucchini. The best approach is to have a reliable Australian reference book to identify problems. Garden Pests, Disease and Good Bugs by Denis Crawford is recommended.

A general strategy to follow with any problem is to observe the plant carefully before taking action. Identify the problem then carefully consider the course of action. For example, with pest insects, you often can see a predator nearby. If you watch and wait the predator may deal with the problem for you. Beneficial predatory insects which feed on pest insects can be purchased from suppliers such as Bugs for Bugs. If the problem needs further action the above recommended book has suggestions.

Practicing good garden hygiene, crop rotation, and maintaining plant vigour by providing adequate nutrients are key strategies for prevention.

Common issues include:

Wilting leaves during the day. It is common for the leaves to wilt during the day. As long as they perk up again in the evening it is not a problem. If not you need to give them more water.
Poor fruit development If fruits start to develop but then wither or blacken this generally indicates insufficient pollination, which can be an issue early in the season when fewer pollinators are active. To improve pollination, you can hand-pollinate by transferring pollen from male flowers (with thin stems) to female flowers (with small fruits at their base) using a small brush.
Powdery mildew: This fungal disease appears as white powder on leaves. Ensure good air circulation by proper spacing, avoid overhead watering, and apply milk spray (1 part milk to 9 parts water) or seaweed spray as a preventative measure. Select varieties that are less prone to powdery mildew.
Downy mildew: Yellow spots on leaf surfaces with grey-purple fuzz underneath. Improve air circulation and apply organic copper fungicides if severe.
Bacterial wilt: Caused by cucumber beetles, plants wilt suddenly and don’t recover. Remove and destroy affected plants and practice crop rotation.
Cucumber beetles: Small yellow and black striped beetles that feed on leaves and spread disease. Protect young plants with floating row covers until flowering.
Squash bugs: Gray-brown shield-shaped bugs that suck plant sap. Check undersides of leaves for bronze-coloured egg clusters and crush them. Remove adults by hand.
Vine borers: White larvae that tunnel into stems, causing wilting. Look for sawdust-like frass at entry holes. Cut out of vines and hill soil over the wound.

Recommended Varieties

Several zucchini varieties perform well in Canberra’s climate:

Striato d’Italia: Sweet and nutty flavour with attractive striped fruit. Plants are vigorous and produce throughout the season. Fruits grow long before increasing in girth. Resistant to powdery mildew.
Rond di Nice: Round zucchini with excellent flavour, particularly good for stuffing or grilling. Earlier to produce than some varieties. Resistant to powdery mildew.
Black Beauty: Traditional dark green zucchini with straight fruits and reliable production. Prone to powdery mildew.
Cocozelle: Italian heirloom with striped dark and light green fruits. Compact plants suitable for smaller spaces. Resistant to powdery mildew. Seemingly less productive than some other varieties.
Golden: Bright yellow fruits with a slightly nutty flavour and softer skin than green varieties e.g. yellow crookneck, Gold Rush.
Romanesco: Distinctive ribbed fruits with excellent flavour and firm texture. Resistant to powdery mildew.

References

Bubel, Nancy. The New Seed-Starters Handbook. Rodale Books, 2018.
Crawford D. Garden Pests, Disease and Good Bugs. Australia: ABC Books; 2015.
Dove, Helena. Botany of the Kitchen Garden, Kew Publishing, Royal Botanic Gardens Kew, UK, 2023.
Fanton, Michel and Jude. The Seed Savers Handbook. The Seed Savers Network, Australia, 1993.
Gardenate. Growing Squash (Crookneck, Pattypan, Summer squash) in Australia – temperate climate [Internet]. 2024 [cited 2025 May 6]. Available from: https://www.gardenate.com/plant/Squash
The Seed Collection. How to Grow Summer Squash Seeds [Internet]. 2024 [cited 2025 May 6]. Available from: https://www.theseedcollection.com.au/growguide/summer-squash-seeds
Yates Australia. How to Grow Squash [Internet]. 2024 [cited 2025 May 6]. Available from: https://www.yates.com.au/how-to-grow/squash/