Growing tomatoes
Tomatoes need a sunny and dry spot, well-protected from the wind. Growing tomatoes outside in open soil can be a gamble because, during a bad summer, they often suffer from tomato disease and have difficulty ripening.
Growing tomatoes in a greenhouse yields the best results. If you don't have a greenhouse, you can opt for a mini-greenhouse or make a shelter with some corrugated sheets. It's especially important that the tomatoes don't get wet from rain. There are also special tomato covers available, which create a microclimate and protect against harmful insects like whiteflies. However, during heavy rain, these covers can backfire by promoting mold growth.
Tomatoes should be grown in the fruit area of the garden. This is important to keep in mind for proper crop rotation.
How to Grow Tomatoes
When to sow tomatoes
If you want to sow tomatoes yourself, you need a place with enough light and warmth (at least 20°C). You can start pre-sowing tomatoes indoors from March without needing additional lighting or heat. After a week, you'll see seedlings appear.
Outdoor sowing can only begin from the end of April because there's still a risk of frost before then. The downside of late sowing is that your harvest will be later as well. Therefore, it's advisable to pre-sow indoors or in a greenhouse so that you can enjoy your tomatoes sooner.
Hoe je best zaait, lees je hier.
Transplanting tomatoes
Once the tomato seedlings are 5 cm high, you can transplant them into 10 cm diameter pots. Use good potting soil, mixed with compost or worm castings. Plant the tomato seedlings deep enough so they can develop many roots.
When to plant tomatoes outside
Six to eight weeks after sowing, you can transplant the tomato plants (about 20 cm tall) outdoors.
Before planting them outside, they must be hardened off to handle temperature differences.
Tip: Don't plant tomatoes close to potatoes as both are susceptible to the same fungus (Phytophthora).
To plant tomatoes in the greenhouse, dig trenches first. This simplifies watering later on. This can also be done outdoors. Water the tomato seedlings well before planting and place them in the planting holes, lightly pressing the soil. You can also mix worm castings and regular garden soil in the planting hole.
How far apart to plant tomatoes
The distance between tomato plants is 40 to 50 cm between plants and 75 cm between rows.
Growing tomato plants in pots
It’s possible but yields less than tomato plants grown outdoors in open soil. Cherry tomatoes provide the best yield in pots.
Tomato fertilizer
Tomatoes need a lot of nutrients to grow. Half-decomposed compost and worm castings can be spread as mulch around the trenches of tomato plants. You can also fertilize tomatoes with cow or horse manure, but always use decomposed manure.
Mulching with grass or other plant residues during the growing season can serve as additional nutrition for soil life and help prevent weeds.
If your soil is well-prepared with organic matter, you don't need granular fertilizers. You might consider adding wood ash for extra potassium, but it’s not essential.
If your soil isn't yet ideal, you can feed tomato plants with a plant-based fertilizer like Vega N7. Tomato plants in pots lack sufficient reserves and need to be watered with a liquid fertilizer every 14 days, such as Vega N7.
Tomatoes don’t need lime specifically; the soil should have a neutral pH for all vegetables, neither too acidic nor too alkaline.
Staking tomato plants
Tomato plants can't grow without support unless you're growing dwarf tomatoes. There are various techniques depending on where the tomatoes are planted:
- If you're growing tomatoes in pots, it's best to use bamboo stakes. Three bamboo stakes per plant are sufficient, and if needed, tie a rope around the stakes so the tomato plant doesn't fall over.
- You can use a tomato spiral, suitable for both open soil and pots. Place the spiral next to the plant, deep in the ground. As the plant grows, twist it around the spiral. Be careful not to twist too quickly, or you might break the top.
- In a greenhouse, you can grow tomatoes by tying them to a string. Attach the string to a horizontal wire stretched across the length of the greenhouse.
Caring for tomato plants
Pollination of tomato plants
You can improve pollination by placing a bumblebee box, especially useful for pollinating the first clusters in a greenhouse where bees and bumblebees are less active early in the season.
You can also manually pollinate by tapping the strings weekly to release pollen. Pollination failure is often due to low temperatures or high humidity (ideally 18°C with 65% humidity). Outdoors, wind usually ensures proper pollination.
Tying tomato plants
This should be done weekly, as tomato plants grow quickly.
Proper ventilation is crucial to prevent mold. Tomatoes dislike humid air. During summer months, you can leave the greenhouse open at night.
Watering tomatoes should be done in the morning if necessary, so the plants have time to dry during the day. Overwatering can lead to shallow root systems. Ensure the soil is well-moistened before planting, and you can often wait a month or longer before needing to water again.
Strengthening tomato plants
Spraying the plants with Oenosan every three weeks strengthens the leaves, making them less susceptible to mold. Thanks to Oenosan, you may also need to water less during the summer months.
Mulch with worm castings, compost or with pruned tomato leaves so that the ground does not dry out too quickly.
Tomato suckers
Suckers that develop between the main stem and side branches should be removed weekly. These take energy away from the primary fruits.
Topping tomato plants
You remove the head from the plant and cut after a few flower clusters. How high should you let your tomato plants get?
In a greenhouse, you can leave 5 to 8 flower clusters before topping. When growing outdoors, you should top after only 3 to 5 flower clusters, depending on how hot or drizzly the summer is. If you don't do this, it comes at the expense of yield. You cut away the top two leaves above the last cluster. In the upper leaf axils, thorns will grow back. Of these, you leave one so that the sap flow continues to the top.
Pruning leaves: which leaves of the tomato plant should be removed? The lower leaves of the tomato plant are picked away at the moment the lower bunches are well formed. By doing so, you bring more air and light between the plants which promotes the ripening of the first fruits.
Pick away 2 to 3 leaves in several turns.
Watch out: if you are using Encarsia parasitic wasps or Eretmocerus against whitefly, then always leave the picked leaves in the greenhouse for several weeks. That way the pupae of the parasitic wasps located on the underside of leaves can still hatch. Otherwise, you would discard them to the compost pile!
When to harvest tomatoes
You can harvest tomatoes as soon as they reach their full color. That can be yellow, orange or red, depending on the variety. There are also green varieties of tomatoes nowadays.
If you pick almost ripe tomatoes, they can continue to ripen indoors, but they taste best when they ripen fully on the plant. If you want to make juice from your tomato harvest, it is even better to let the tomatoes over-ripen on the plant a bit; this is when they give the most juice!
Re-ripening is definitely an option at the end of the season. Tomatoes in a cold greenhouse can tolerate light night frost (-1 -2 °C) before freezing. On sandy soil, they will freeze faster than on heavier soil types.
Outdoor tomatoes can be left to ripen by pulling out the plants and hanging them upside down. Closet tomatoes are placed in containers covered with newspaper and placed in the cellar or in a cool room. Re-ripening can take several days to several weeks. Check regularly for rotten tomatoes, especially if they have endured overnight frosts.
Tips for growing tomatoes
Sowing tomatoes from tomato
You can find many techniques on the Internet for drying seeds from a tomato and sowing them later. I myself received a tip years ago from a follower who recommended cutting a nice, ripe tomato into slices and letting them dry in the sun. You simply place the dried slices in a container with potting soil the following year. You cover the slices with a thin layer of potting soil. A very simple and successful way to grow new tomato plants from your own seed.
Overwintering tomatoes
This is not often done. Although tomato plants are perennial, they are not resistant to frost. Because tomatoes are usually grown outdoors or in a greenhouse, overwintering is often not an option. Moving large tomato plants from outside to inside is also inconvenient, and transplanting them again the following year makes it even more difficult. In my opinion, it is not worth the effort and you are better off starting the new season with fresh tomato plants.
If you have tomato plants raised in a warm greenhouse, overwintering is possible, but the plants are often more quickly attacked by harmful insects such as white fly and aphids. Good monitoring and quick release of beneficial insects is then the message.
Tomato cuttings
You can easily take tomato cuttings in water. To do this, use the shoots, also called “thieves,” that grow in the axils between the main stem and side stem. Cut off the thief with pruning shears and put them in a pot of water. Taking tomato cuttings makes most sense if you do it in late fall so you can overwinter the cuttings. Then in the spring you won't have to sow tomatoes and you'll have faster plants to plant out in the greenhouse. Propagating special tomato varieties in this way is definitely a bonus.
Diseases on Tomato Plants
Here, we discuss the most common diseases that can occur:
Phytophthora in tomatoes is commonly known as the blight; this fungus affects both tomatoes and potatoes. It presents the same symptoms in both plants. On leaves, stems, and fruit stalks, dark brown to black spots appear.
The fruits affected by Phytophthora develop brown spots on the outside, while the tomatoes have dry, rotten areas inside. Phytophthora is the most important disease for outdoor tomatoes but can also occur in greenhouses if there is too much humidity or excessive watering.
Biological Control of Phytophthora
- Always ventilate a greenhouse or plastic mini greenhouse well. Outside, cover the tomato plants with a shelter.
- Spray Oenosan plant-based fertilizer every three weeks on the leaves of growing plants. This strengthens the leaves, making them less susceptible to fungal infections.
- Quickly and carefully remove the first infected leaves or stems. Do not compost them but dispose of them through the organic waste bin.
Young plants may develop brown spots on the stem and die off; this is known as botrytis rot.
On larger plants, rotten, brown spots with grayish-brown fungal fluff appear. This is leaf and stem botrytis.
On green fruits, you can usually see small light-colored rings with a dark spot in the middle, called botrytis stip. Over time, these spots grow larger, and the fruits fall off.
Fighting botrytis on tomatoes:
- Maximize ventilation and reduce watering. Remove leaves from dense foliage. Spray Vega N7 plant fertilizer on the leaves every three weeks.
- Limit watering by using Oenosan.
This fungal disease is also called stem cancer or foot rot and is typical for outdoor tomatoes. Brown spots appear on the stem near the ground. The leaves turn yellow and die off. Fruits develop brown or black round spots. The disease spreads through the soil, seed, and support materials.
Fighting cancer on tomatoes:
- Do not plant tomatoes in cold soil and ensure proper crop rotation. Remove sick plants immediately and dispose of them through organic waste bins.
Whitefly is one of the most common pests in greenhouses.
The larvae and adult whiteflies suck sap from tomato plants. The excess sugars they ingest are excreted, creating a sticky substance that dirties the tomatoes. After a while, a black sooty mold grows on this honeydew.
Whitefly can overwinter in various stages: as larvae, eggs, or adult insects in a greenhouse. Treatment is necessary to prevent them from reappearing early the following season.
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Hang the yellow sticky traps in the greenhouse to detect and catch the first whiteflies. Plant marigolds (*Tagetes*) or a tobacco plant in the greenhouse, as their scent may help repel whiteflies.
Whitefly on tomato plants
Use Encarsia sluipwespen or Eretmocerus parasitic wasps to combat whiteflies. Start treatment at the first sign of infestation to quickly bring the pest under control. The Swirskii-system (slow-release predatory mite sachets) is another option for fighting whiteflies. However, Swirskii predatory mites require high temperatures (average of 25°C) and are not suitable for use in early spring.
Note: Whiteflies can also overwinter on weeds or on potted plants that you bring into the greenhouse during winter. Ensure the greenhouse is weed-free if you've had a whitefly problem, and make sure any potted plants are insect-free before overwintering them.
Greenhouse spider mites are very small, beige-brown in color, with two dark spots on the sides of their bodies. They suck sap from the underside of the leaves, causing yellow spots to appear on the upper surface. Over time, the leaves become discolored, and webs form between the leaves.
Preventing spider mites in tomatoes:
Ensure good ventilation, but avoid really drafty areas.
If you struggle with spider mites every year, use Soni-mite slow-release sachets preventively once the plants are in the greenhouse.
Combating spider mites in tomatoes
Use Phyto-mite to control spider mites in the early stages of an infestation. Alternatively, if the infestation gets out of hand, you can use Chrysopa larven.
Blossom end rot is typically caused by calcium deficiency in the plant. This can be due to inconsistent watering, high salt concentrations, or even an imbalance in fertilizer use. If the issue persists, a soil test is recommended to understand the root cause better.
Leaf curling in tomato plants can occur for several reasons, and it isn’t always a negative sign:
- Slight curling at the top of the plant can indicate strong, healthy growth.
- If the leaves around the stem are curling, it may suggest overly vigorous growth.
- If young leaves are curling, check the underside of the leaves for aphids.
- Lower leaves curling and drying up is normal and usually not a concern.
Ripe fruits crack easily when they receive too much water. Therefore, be cautious with watering and pick the fruits before watering. The transition from warm temperatures in late summer (around mid-August) to colder nights can also cause tomatoes to crack, especially if you continue giving them plenty of water. Ideally, after mid-August, water should be reduced significantly. By mid-September, you should stop watering altogether, even if the weather remains warm.