Tomatoes

Start from seed: August-September

Optimum soil temperature for germination: 24-32⁰C

Plant seedlings: November-December

Plant spacing: 40-50cm

Days to maturity: 45-87

Introduction

Tomatoes, belong to the Solanaceae family, which also includes eggplant, tomato and chillies. Membership of this family, that has highly poisonous members such as deadly nightshade and tobacco, led to suspicion when tomatoes were first introduced to Europe. The tomato has been known as the “love apple” due to its association with the aphrodisiac herb Mandrake.

Tomatoes have been recorded as growing in the wild in Peru and it is thought that modern tomatoes evolved from these wild cherry types. They were domesticated in the Andes and Mexico and brought to Europe by the Spanish in the early 16^th century. Towards the end of the 18^th century, they were brough to the United States of America. By 1830 tomato ketchup was “national condiment” in the USA and by the end of the 19^th century large scale tomato canning was practiced in Italy and the USA.

Tomato comes from the Aztec word Tomath. The plant is grown as an annual and has an erect growing habit with hairy stems and deeply lobed leaves. The start shaped yellow flowers are self-fertile which means the seeds grow true to type. Botanically the fruit is a fleshy berry that comes in a huge variety of colours, shapes and sizes.

Site and soil

Tomatoes can be successfully grown outdoors in Canberra. Being a fruiting plant, tomato requires at least 6 hours of full sun per day. Choose a warm, sunny, ideally north facing, site.

Prior to planting add compost and a complete organic fertiliser to the soil. They like a fertile, well-drained soil with pH of 6.5.

If growing in pots it is very important to boost the potting mix with complete organic fertiliser.

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

If starting tomato from seed this should begin in mid-August. The optimum germination temperature for tomato is 30⁰C. Therefore, the most successful method is to plant seed into cell trays or trays and provide bottom heat by using a heat mat or hot water bottle wrapped in newspaper or a tea towel. Warm, sunny places inside near a window are also suitable for germination. To maximise germination a soil temperature probe and thermostat are useful to ensure the soil temperature is maintained between 24-32⁰C. Germination will occur without heat but is likely to be much slower.

As soon as the seeds have geminated, they need full outdoor daylight or the equivalent provided by grow lights. Once the seedlings have developed true leaves, and are around 4-5 weeks old, they will grow better if transplanted into a pot to give the roots more space and allow them grow on before planting outdoors. Advanced plants will fruit earlier. Remember to protect your seedlings from the cold.

Prior to planting, all seedlings should be hardened off. This process takes about one week and involves gradually increasing the length of time that the plants are exposed to full sun. If this is difficult to do prior to planting, an alternative is to make a shade cloth cover over the garden bed containing the tomatoes and gradually reduce the amount of cover over a week or two.

Plant seedlings approximately 40-50cm apart. After planting, it is a good idea to water them in with a seaweed solution. The soil and plants will benefit from being mulched with organic matter such as spoiled lucerne, pea straw, sugar cane mulch or leaf mould.

Cultivation

Since tomatoes are cold sensitive, seedlings, whether purchased or raised yourself, should be planted out after the danger of frost has passed and the soil is warm. In Canberra this is usually sometime in November. If the overnight temperature is forecast to be 2⁰C or less cover plants with frost cloth or an upturned bucket to protect from frost. Covering plants can make a difference of 5⁰C. The ideal temperature for fruit set is between 18-29⁰C. Very little fruit set occurs below 16⁰C and above 32⁰C. Covers can be used to increase yield and extend the season into late Autumn.

If seedlings do not “take off” after a couple of days a foliar (leaf) spray can be very beneficial. This involves spraying the leaves of the plants in the early evening with a solution of seaweed, seaweed and worm juice or a trace element-based product. You should notice an improvement in the plants with 24-48 hours after applying a foliar spray. Feeding plants in this way stimulates plants to produce carbon rich root exudates which feed soil microorganisms who in turn provide mineral nutrients to the plants and can be done monthly during the growing season.

Chilling can induce early flowering. This is called vernalisation. It must be done when the first true leaves are opening up and the plant is 4cm tall. Night time temperatures of 10-13⁰C for 2-3 weeks is effective.

Weeds can compete with crop plants, therefore weeding at the seedling stage is beneficial. Mulching will supress weeds to some extent. Weeds will be less problematic once plants form a canopy shading the soil.

Staking is not absolutely necessary, however, it makes the plant easier to manage, keeps them upright, maintains air circulation, helps reduce disease and keeps the fruit off the ground. Tomatoes can also be trained to grow up strings suspended from above, or trained on a trellis, such as basket weave trellis.

Tomatoes can be vigorous growers and will produce lateral (side) shoots at every node. The node is where the branch joins the stem. These lateral shoots should be gently pinched off or pruned off with tomato pruners or secateurs. If they are left you can end up with plants with a significant amount of vegetative growth at the expense of fruiting. Abundant laterals make the plant hard to manage. Alternatively, you can let tomatoes follow their natural habit. You will still get fruit but you may have to battle foliage to get to it.

Plants may need to be fertilised mid-season using a side dressing of complete organic fertiliser.

Water is the number one requirement for plant growth. Newly planted seedlings will generally need to be watered once or twice a day. The easiest way to gauge whether plants need watering is to insert a finger into the soil. If it is wet and soil adheres to the finger it does not need watering. If it feels dry it needs watering. Soil should feel cool and moist. Try and maintain soil in this condition by regular watering.

Harvesting

Harvest fruit when it reaches the desired colour. Fruit in Canberra will probably not get as large as tomatoes you are used to seeing for sale so do not worry if the fruit is small. Late in the season they may not ripen to red, orange or yellow.

Fruit can be gently removed from the plant using a gentle upward pull or they can be cut off with secateurs or a sharp harvest knife. As is usual with fruiting plants, harvesting regularly will stimulate more flowers which means more fruit! Tomatoes can continue producing well into autumn.

Problems

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

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. A four-to-five-year rotation for all Solanaceae crops is recommended.

Here are a few common issues with tomato.

Blossom-end rot affects tomatoes. The symptom is brown, sunken, watery areas at the bottom (blossom) end of the fruit. The cause is an inadequate supply of calcium resulting in plant tissue break down. Inadequate watering, waterlogging, over use of fertilisers or low soil pH can all reduce the availability of Calcium leading to this condition. Prevention involves maintaining a soil pH of around 6.5, deep and regular watering, using organic fertilisers, ensuring your soil is not Calcium deficient, mulching plants and avoiding severe pruning. Some varieties seem more prone than others to Blossom-end rot.
Sunscald is the equivalent of sunburn. Fruit develops papery blisters which can lead to secondary infection and pest infestation. It can be avoided by selecting varieties with good leaf cover or by shading plants in periods of intense bright light.
Tomato russet mite. Affects tomatoes but can also affect tomatoes. If the leaves start dying off from below, curl downwards, dry out and turn bronze, and the stems lose their hairs, get out your magnifying glass and see if you can see the tiny mites above the damaged part of the plant. The mites move up the plant creating havoc on their way. Predatory mites are a natural predator. A solution is to spray with wettable sulphur, lime sulphur or tomato dust. The odour from the sulphur kills the mites. It is important to act quickly on this one as plants can die within a week. At the end of the season keep affected plants and mulch out of the compost to break the life cycle.

Queensland fruit fly has become a problem in Canberra mostly affecting larger fruit late in the season. Exclusion using fine netting over plants or around individual fruits is successful. Traps, lures e.g. Fruition, and Spinosad sprays are also effective when used together.
Tobacco Mosaic Virus Tomatoes are vulnerable to diseases spread by tobacco users such as Tobacco Mosaic Virus so it is wise to avoid tobacco users from handling tomato plants.
Avoid planting tomatoes near walnut trees because they are extremely sensitive to the toxin Juglone which is exuded by walnut tree roots.

Recommended Varieties

There are two types of tomatoes: determinate and indeterminate. Determinates are also known as bush tomatoes. They grow upwards, then stop and have a limited number of flowers resulting in all the fruit ripening at the same time. Indeterminates continually grow upwards and produce trusses of fruit along stems throughout the growing season. Their growth habit resembles that of a vine which is why they can be easily trained to grow up a string.

Since there are over 10 000 varieties available and everyone has their favourites, it is hard to recommend specific varieties. If fruit fly is a problem grow cherry style and small tomatoes and avoid growing large fruited varieties.

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.