You can use soap spray as an insecticide on tomatoes at any time, up to the day of harvest.

Can I Use Soapy Water Spray on My Tomatoes?

by Veronica Smith-Jennings

If you're looking for an effective insecticide that won't hurt the environment, add toxic ingredients to the food you're growing or break the bank, you might find it in your own kitchen. Mild soaps used for hand washing or gently washing dishes often have insecticidal qualities that eliminate harmful pests. You can create a solution and apply it to your tomato (Lycopersicon esculentum) plants to fight plant-eating insects; soaps can wash off the insects' protective outer coating or immobilize them so they wash off the plant. Other insects eat the soap along with the plant and die. A spray made with gentle soap does not hurt your plants or harm the rest of your garden, and is safer to store in the house. As with all insecticides, though, make sure it's stored where no pets or children can get to it.

Soapy Water Can Help

If you see your tomato plant's foliage disappearing thanks to feasting insects, give a soap spray a try. Whether it's flea beetles or aphids eating tiny holes in the leaves, the soap spray can help. Other small, soft-bodied pests that damage your plants, such as whiteflies and spider mites, can also be eradicated by soap sprays. Larger insects tend to remain unaffected, although several, including the Japanese beetle, are vulnerable to soap sprays. Most beneficial insects, including honeybees, will not be hurt by these sprays.

Best Soaps for Plants

Rather than buying expensive products, make your own soap spray for your tomato plants. You can find commercially produced soaps -- these are produced specifically to combat insects and to be safe for plants. But you can make your own batch, too, with household products. Use hand soap or mild dish soap -- powdered soap, laundry detergent and dishwasher detergents are too harsh. Mix the liquid soap with water to create a 2 to 3 percent solution. For a 2 percent solution, mix 5 tablespoons of soap into a gallon of water. Increase the soap to 8 tablespoons for a 3 percent solution. Soft water is best -- use distilled water if you are uncertain about your tap water's hardness level. Put the mixture into a spray bottle for easy application.

Application Tips

The soap spray needs to cover as much of the tomato plant as possible for maximum effectiveness. Spray all the leaves, back and front, with the solution. The spray needs to cover the insects to work, so you need to find wherever they might be hiding within the foliage. As the spray dries, it loses potency. You can reapply the spray every four to seven days until the insects have left the tomato plants.

Keeping Plants Safe

Before you apply any homemade soap spray to your tomato plants, test it on part of one plant. Then wait two days to make sure it has done no damage. Certain species of tomato plants may be more prone to damage by soap sprays, so it's safest to ensure your soap solution isn't too harsh. You can also wash the solution off of the plants soon after application to ensure it does no damage to the leaves, or try a less potent solution.

About the Author

Veronica Smith-Jennings is a former teacher who started freelance writing in 2003 and has been published in regional parenting magazines as well as on various websites. Her writing interests include home renovation and gardening, politics, education, sports and early childhood development. She has a Master of Arts in English education and a Bachelor of Arts in English.

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