How To Grow Tomatoes From Seed

Tomatoes are one of the most popular garden vegetables - and not surprising since the taste of fresh tomatoes from the vine is divine. Growing tomato seeds is a highly rewarding experience. By growing your own tomatoes, you’ll have tomatoes tastier than any shop-bought varieties you can find.

There are numerous varieties, from small-fruited cherry tomato seeds to monster beefsteak tomato seeds. These can be found in a spectrum of colours, ranging from standard red to yellow, orange, green, purple and striped. There are also the standard tall cordon varieties to bush and even hanging basket types. Although growing bags are the favoured growing medium, the plants take a lot more careful looking after than those growing in pots or in the ground. Outdoor tomatoes are also well worth growing.

To learn in further detail about how to grow tomatoes from seed, this complete guide contains all the instructions you need to grow tomato plants bearing fruit that’s packed with flavour.

Determinate vs Indeterminate Tomatoes

Tomato plants are distinguished into three different types: indeterminate (cordon), determinate (bush) and semi-determinate tomatoes. They are differentiated as follows:

Indeterminate (cordon): Indeterminate varieties grow vertically, requiring a lot of space as they will grow very tall. They will need more maintenance in the form of supports and regular watering, feeding and pinching out of side shoots. These produce fruit continuously until frost.

Determinate (bush): Determinate varieties are smaller and more compact plants that grow in a sprawling manner. Their size makes them a preferred option for those limited on growing space, being highly suited to hanging baskets and pots. They also don’t require much maintenance other than regular watering and feeding. These will stop producing fruit once the plant reaches a particular size. 

Semi-determinate: These plants are a middle ground between the indeterminate and determinate tomatoes. They will grow higher vertically than determinate varieties, though will not continuously grow as an indeterminate variety would.

Their different methods of growing hold different appeals for different growers. For those looking for a smaller, low-maintenance plant, determinate varieties are the better-suited option. For people wanting a continuous harvest and are willing to consistently prune and care for their plant, indeterminate varieties are a great choice. 

Semi-determinate varieties make for a convenient offering in having the advantages of both, producing fruit over a longer period than determinate varieties but being more manageable than indeterminate varieties.

Tomatoes are often grown in greenhouses or indoors to keep them at the temperatures needed for healthy growth. Indoor tomato seeds are typically grown in plastic grow pots or growing bags. However, outdoor tomato seeds can be grown outside in warmer months.

If growing in containers, they are best grown in peat-free garden compost. If you’re growing outside, the soil should be rich, well-drained and moisture-retentive, adding in plenty of compost, manure or fertiliser before planting.

You should not grow tomatoes in soil that has previously been used to grow tomatoes, as this could lead to your plants being affected by diseases.

When to Sow Tomato Seeds

For greenhouse cultivation, tomato seeds can be sown from mid-January to early February (heated greenhouse) or late February to mid-March (unheated greenhouse). For outdoor cultivation, sow tomato seeds in late March to early April.

How to Sow Tomato Seeds

Sow tomato seeds at 18°C (65°F) in seed trays or small pots. Using a heated garden propagator or placing potted seeds on a warm windowsill covered with a clear plastic bag will help to maintain the moisture and temperature needed for germination. Your seeds should germinate between 7-14 days.

Once seedlings appear, they should be uncovered and placed in an area with plenty of light to prevent them from becoming leggy. When two true leaves have formed, prick your seedlings into 3.5in (9cm) pots.

When the flowers of the first truss begin to open, transfer your tomato plants to 9-inch (23-cm) pots or growing bags if growing indoors or in a greenhouse. Alternatively, plant them 18-24 inches (45-60cm) apart outside in a sunny, sheltered spot after the risk of frost has passed.

Plants should be hardened off before planting outdoors. This involves gradually exposing them to outdoor conditions, leaving them outdoors for 1-2 hours and increasing their time spent outdoors between 1-2 hours each day over a period of 7-10 days. After about a week, they can be left out overnight if temperatures are above 10°C (50°F).

When ready for planting, tie the main stem of indeterminate varieties to a vertical bamboo cane or wind it up a well-anchored but slack, sturdy string. Determinate varieties generally don’t require support unless they become especially large.

Semi-determinate varieties, such as Tomato Marmande AGM seeds, will grow in a bushy manner similar to determinate varieties but will require support like indeterminate varieties due to growing a little taller.

Watering

Water your plants regularly to keep the soil evenly moist, but not so much that they become waterlogged. Plants grown in pots will require frequent watering as they are prone to drying out faster, especially on hotter days. Take care to avoid getting foliage wet, as this can lead to the development of diseases.

Mulching

Mulching your tomato plants can help to regulate the moisture and temperature of your plants as well as prevent weed growth. Therefore, it’s a highly worthwhile step when growing tomatoes. To mulch your tomato plants, add a layer of compost or well-rotted manure, leaving space around the stem to prevent your plants from rotting.

Feeding

Tomatoes are heavy feeders, so require a good amount of fertiliser during their growth. For best results, we recommend feeding tomato plants every 10-14 days with a balanced liquid fertiliser, such as D.T. Brown's Plant Tonic No. 8. You should then switch to a high-potash fertiliser, such as D.T. Brown’s Tomato Fertiliser and Biostimulant No 7, once the first fruits start to set.

Pruning

Remove yellowing leaves below developing fruit trusses. Once indeterminate plants reach the top of the greenhouse, have set seven trusses indoors or have set four trusses outdoors, remove the main stem's growing point at two leaves above the top truss.

You should remove side shoots regularly from indeterminate varieties when they are about one inch (2.5cm) long. This helps to focus energy on producing larger fruit on their single stem. However, they should be left on determinate varieties, as this helps to maximise fruit production in their shorter growing period.

Start picking when the fruit is ripe and fully coloured. This will generally be from mid-summer onwards, though it depends on the weather conditions, variety and fruit size.

At the end of the growing season, lift the plants with unripe fruit and either lay them on straw under garden cloches or hang them in a cool shed to aid ripening. Alternatively, you can pick the green fruit and store it in a drawer next to a banana. Bananas release ethylene gas, which is a natural hormone that encourages the ripening process.

Tomatoes shouldn’t pose many problems when looked after properly. There are some issues plants have been known to experience, but most can be easily prevented or remedied. Below are some of the most common tomato problems and methods of dealing with them:

• Whitefly - Tomato plants can be susceptible to attacks from whitefly. Using pest and disease control products, such as whitefly killer, can help to resolve the problem.
  
  
• Fruit problems - There are various problems that are known to affect tomato fruit. Some of these include splitting fruit, “greenback” (when fruit fails to ripen properly and develops hard green areas on the ‘shoulder’ of the fruit) and blossom end rot (where the fruit develops dark blotches on its ends).
  
  Most of these problems are preventable by simply keeping your tomatoes well looked after. By consistently watering and feeding your plants as well as preventing exposure to extreme temperatures and high humidity, your tomato plants should grow healthily and produce well-formed fruit. 

How Do You Prevent Blight on Tomato Plants?

Tomato blight occurs when the right conditions are provided for the fungal-like disease to grow. This is when your tomato plants are exposed to very high levels of moisture and very warm temperatures. Symptoms will appear in the form of black or brown spots on foliage and eventually cause your plants to shrivel and rot. The disease typically infects outdoor tomatoes and rarely affects indoor or greenhouse-grown plants.

For gardeners who have previously had their crops affected by blight or those who would like to avoid the risk altogether, blight-resistant tomato seeds are an excellent choice. These tomato seeds offer natural resistance to the disease, meaning that they are significantly more likely not to be affected. However, there is still a very minute risk that the disease could affect them. However, with the right care, you can prevent blight from affecting all of your crops, blight-resistant or not.

You can prevent tomato blight by:

• Spacing out your plants - This increases airflow and reduces blight risk, so give your plants a good amount of space between each other.
• Avoid watering over leaves - Water directly onto soil and avoid getting foliage wet.
• Avoid replanting in previously affected soil - Blight spores can remain in soil and reaffect plants that are planted afterwards. Be sure to practise crop rotation with soil unaffected by blight to prevent its occurrence.
• Use high-potassium fertiliser - Using nitrogen-rich fertiliser encourages leaf growth and increases the risk of blight, so use a specialised fertiliser, such as D.T. Brown’s Tomato Fertiliser and Biostimulant No 7, instead.

Destroy affected plants - Either deeply bury (below cultivation depth), burn or put affected plants into your green bin. Do not compost them, as this could reinfect other plants.

Now that you know how to grow tomatoes from seed, from sowing to harvesting, you can get started by shopping our expansive range of tomato seeds and tomato plants. We have over 100 years of experience in providing gardeners with excellent quality seeds at D.T. Brown, so you can rely on our choices to produce tomatoes with exceptional flavour.

If you’d like to learn how to grow a variety of other vegetable seeds and vegetable plants, our garden blog has many other thorough growing guides similar to this one to get your garden flourishing.

To find out more about our range offruit plants, fruit trees, flower seeds, or anything else in our selection, don’t hesitate to get in touch and we’ll happily answer any questions you may have.