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Once planted, apply a layer of black mulch (3 to 6 inches (7.6 to 15.2 cm)) on top of the soil around your tomato plants. You can add mulch to both gardens and pots. The mulch will keep the soil moist, but will also help control weeds and prevent ground rot.  You can use commercial mulch that comes in plastic bags from a garden centre, or you can use straw. When laying the mulch on the ground, keep a 1 in (2.5 cm) space around the stem of your tomato plants to prevent fungal problems. Watering your tomato plants frequently and lightly can cause tomatoes to crack or the blossoms of the plants to rot. It can also allow the development of foliage disease. Soak the soil around your tomato plants every 7 to 10 days instead. When soaking your tomatoes, spray the undersides of the leaves with your hose to remove any initial spider mites that may have started to make a home.  If the soil around your tomato plants is drying out before your next watering day, increase the frequency of the soaking. Tomatoes planted in pots may need to be soaked more frequently, as pots tend to dry out faster. The spray from an outdoor hose is of a high enough pressure to help keep spider mites from being able to make a home on your tomato plants. Spray your tomato plants with a hose, especially on the undersides of the leaves, on a weekly basis. This spray will help keep spider mites away and will also water your tomato plants.  This method can be used on tomatoes planted in a garden and planted in pots. Be careful that the spray of water isn’t too strong that it damages the tomato plants. You may want to use the nozzle and spray upwards on the plants to get the undersides of the leaves. Or, hold the leaves with your hands and turn them over before spraying them. Spider mites hate moist conditions. To help prevent spider mites on your tomato plants, ensure there is always a layer of water in the tray or saucers in which your tomato pots or containers are sitting. This water will keep the soil moist, which in turn, should help keep spider mites away.  This method will only work for tomato plants in pots. However, if you put your tomato plants in a garden, you can leave buckets or pots of water in and around your plants to increase the general moisture around your tomatoes. This is especially important when the conditions are hot and dry. Tomato plants that are water-stressed are more susceptible to spider mite infestations. Cluster your potted tomatoes together on a porch or deck and place a humidifier in the middle of the cluster. Keep the humidifier going during the driest parts of the day. If your tomato plants are indoors, put the humidifier anywhere in the same room as the plants. Remember to check the reservoir of the humidifier frequently and keep it full of water.
Use black mulch to preserve soil moisture for your tomatoes. Soak your tomato plants every 7-10 days to prevent rot. Use a hose to remove spider mites from your plants. Keep a layer of water in your pot saucers to increase soil moisture. Install a humidifier close to your tomato plants to increase the moisture level.