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How to successfully grow greenhouse tomatoes in the summer

Expert advice to avoid disease and increase yields



From July/August 2014

greenhouse can be a great place to grow summer tomatoes. The plants often thrive on the extra heat. Greenhouses also help protect plants from disease. Fungal and viral diseases can develop in the field when rain splashes soil onto leaves or when the leaves remain wet for long periods of time. In a greenhouse, the tomatoes won’t (or are far less likely) to be eaten by raccoons. But summertime greenhouse growing has its own set of challenges.

Greenhouse soil

In a greenhouse, you can fine tune the soil’s nutrient balance to meet the needs of your crops. To discover what your soil needs, test the soil before planting and again in the middle of the season.

Ideally, you want to apply soil amendments that provide a slow release of nutrients. This way, the plants can take up nutrients as needed without losing the minerals through leaching. Focus on feeding the soil to feed the plants.

Just as people eat a few meals a day rather than just one huge meal a week, plants do well with frequent access to small amounts of nutrients, according to André Carrier. Carrier is an agronomist with Quebec’s ministry of agriculture, fisheries and food. He recommends that if you are using fertilizers, including organic ones, split the applications to provide smaller amounts of nutrients more often.

For tomatoes, provide just enough compost to meet the phosphorus needs. Use different amendments to provide the other nutrients (such as feather meal for N and Sul-Po-Mag or potassium sulphate for K). A common mistake, says Carrier, is to apply too much compost. In a greenhouse, without rain to leach excessive nutrients away, tomato roots can actually be ‘burned’ by high nutrient levels.

To increase levels of soil organic matter (which provides a slow release of nutrients), add green materials to the soil. For example, you can add fresh hay to the soil and culled leaves. Carrier explains that leaving culled tomato leaves on the soil can supply 25 per cent of the nitrogen needs of the tomato plants. This is “a very wise practice, unless,” he adds, “there is a disease problem.”

Compost tea can be sprayed onto the soil. The objective of the spray is not to add nutrients but rather stimulate soil life, which can then make soil nutrients more available to plants.

In much of Canada, market gardeners often apply lime to their fields and gardens to increase the pH, but the situation is different in a greenhouse. The soil pH can easily become too high because of the lack of leaching and the intensive use of compost and other organic fertilizers that increase the pH. Liming can create an alkaline environment that will tie up phosphorus and other nutrients and stress the plants. Add lime only if the soil test reveals a low pH.


A major challenge of the summer greenhouse is water. You need to find the balance between providing an adequate supply of moisture while avoiding a build-up of excessive humidity.

Good drainage is critical both inside and outside the greenhouse. Poorly drained soil can lead to high humidity levels, which in turn can increase the incidence of diseases such as bacterial canker. You can avoid many problems by installing drainage pipes in the beds and outside the greenhouse.

Many growers use drip tape, which irrigates a strip of soil. For example, in a bed that is four feet wide, use three to four tapes that each cover a one-foot wide strip of soil. Just like with providing nutrients, it is better to split irrigation into different sessions rather than have one long session each day. With drip irrigation, Carrier says, there is no advantage to irrigating for longer than an hour at a time, even outside in the field.? For organic greenhouse vegetables in soil, one to three irrigation sessions a day will suffice.

Begin irrigating when the plants are active, two to three hours after sunrise, and stop two to three hours before sunset. Watering at dawn when tomatoes are inactive can lead to fruit cracking and drops on leaves. Watering at sunset can leave excessive water in the greenhouse and this can lead to fruit and root rot.

Overhead misting systems can be used to change the greenhouse environment. Misting can lower the temperature by 6-8?C. It also can increase the humidity, which is useful when using biological control of spider mites.

Pests and disease

Disease prevention begins with the structure of the greenhouse. “The taller the greenhouse,” says Carrier, “the more stable the greenhouse environment is, in terms of temperature and humidity.” Temperature and moisture fluctuations cause stress in plants, leaving them more susceptible to disease and pest problems. “So,” he concludes, “taller is better.” He recommends sidewalls of at least two metres high.

The plastic should be “anti-drop.” Most greenhouse plastic is treated so that condensation doesn’t form on the plastic. Condensation leads to drops of water that, in turn, may trigger botrytis or other diseases.

While greenhouse tomatoes are free from many of the larger pests that attack outdoor crops, they are vulnerable to insect attacks. In a greenhouse, it’s more challenging to maintain a healthy balanced ecosystem where beneficial organisms (e.g. lady beetles and parasitic wasps) control pests. Growers can avoid many pest problems by ensuring the pests don’t enter the greenhouse.

Screens on the doors and roll-up sides will help block the entry of many pests including tarnished plant bug, butterflies (which produce caterpillars) and cucumber beetles (if you’re also growing cucurbits inside).

Another tactic is to create extreme temperature conditions that will kill many pests. If your greenhouse is empty in late summer, close it up for several days and let the temperatures soar. Or, if it’s empty at some point during the winter, open the doors and sides and let the cold temperatures destroy any pests that might be overwintering. Soaking beds with hot water can also kill pests and disease-causing organisms.

Training and pruning?

Greenhouse space is valuable so you want to maximize the productivity of every square foot. Training plants to grow vertically is one technique. “Tomatoes do well with ‘V-training’ — training at an angle rather than straight up,” says Carrier. The angle will lead to better light penetration into the plant. Also, if the season is long and you need to lower the plant, the stem is less likely to break.

Pruning can increase productivity but every wound creates a risk of botrytis (grey mould). To reduce the risk of botrytis, cut suckers and leaves with a utility knife flush to the stem. If you remove leaves by hand, not a knife, you might rip off a small part of the stem and this can lead to botrytis. Pruning on wet days can also lead to botrytis, so try to leave the pruning until a sunny day.

You can prune in different ways, such as the following.

1. Remove suckers once or twice a week.

2. Remove old leaves but never more than three at a time from a plant. In the summer, keep at least twenty leaves on mature plants.

3. Remove small tomatoes from clusters. For large-fruited varieties, leave three to four fruit per cluster; leave four to five fruit for intermediate-sized varieties.

Greenhouse growing can be a bit trickier than growing tomatoes in the field. A small problem can quickly become out of control in the intensive greenhouse environment. It’s critical to keep monitoring plants and adjusting the environment as needed.

After all, as Carrier says, “We are working with living materials — life — not everything is measured in ppm.”

Intercropping by accident

Near Saint Andrews, New Brunswick, the farmers at Bantry Bay maximize the use of their greenhouse space by intercropping early in the season. They learned this technique by accident.

“We had a cool, wet spring,” said Mike Hadfield, “and couldn’t plant out our cherry tomatoes when they needed to go in.

“Finally we just put the tomatoes in a bed of early carrots in the greenhouse. The carrots were fairly big then and weren’t affected at all and the tomatoes did well. So this taught the value of managing greenhouse space so it’s always planted and sometimes planted in two things.”