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Curing Blossom End Rot
by John Harmon
August 10, 2003

With our on again off again summer it hasn't been a great year for tomatoes. Many folks have asked me what to do about blossom end rot. This condition is where the tomato starts to rot from the point on the bottom where the blossom was attached. Anytime you have weather changing back and forth from cloudy to sunny it stresses your tomato plants. They may not be able to take up Calcium fast enough when it changes back to sunny. The result can be blossom end rot.

Adding extra Calcium can sometimes be the cure but many times the needed Calcium is already in the soil or the fertilizer but the plant just can't get to it. The problem can be that the pH of the soil or growing medium is too high or low. There is a direct relationship between soil pH and the plants ability to take up nutrients. For Calcium it's a fairly narrow band. Anything below six point five or above eight point five will limit the ability of the roots to take up Calcium even when there is an abundance available. (from "Hunger Signs In Crops", edited by H.B. Sprague, 1964,p.18)

The easy thing to do first is check the pH of your soil or growing medium and adjust it as needed. If that still doesn't solve the problem and you're sure the pH levels are good then you can look at adding extra Calcium. I talked about using road salt to add extra Calcium to tomato plants a couple of years ago. Here are the basics again.

Road salt is normally calcium hydroxide, or calcium chloride. Calcium hydroxide is made from commercial hydrated lime and is a dry powder obtained by treating quicklime with sufficient water to satisfy its chemical affinity for water, thereby converting the oxides to hydroxides. Not that I understand this or that it matters to you. I couldn't find any calcium hydroxide around town but I did find calcium chloride and that will work too. Calcium chloride is the stuff the highway crews put on gravel roads in the summer to hold down the dust. It's also used to fill tires for weight.

The part of all this that you should keep in mind is that road salt contains calcium and the lack of calcium in tomato plants can cause blossom end rot. To provide extra calcium to your tomato plants mix just five grams of calcium chloride to a liter of water and spray it on the tops of your plants. James Portree, the B.C. Greenhouse specialist explained the procedure. 'use the spray as a preventative especially in transition weather from cloudy to bright weather and when there is a lot of developing young fruit on the plant (marble size)'. He told me it takes the plants a day or so after cloudy weather to get back up to speed absorbing calcium from the ground or your fertilizer. Just a light spray on the tops of the plants a few times over the summer should be enough to prevent blossom end rot. Jim suggested I try just a few plants the first time around to make sure the solution isn't strong enough to damage the plants. I'm thinking half strength to start with.

I used it half strength at first and then full strength since then and it has worked just fine. Do your spraying on cloudy cool days when the sun can't hit the wet plants. Don't go down and swipe some road salt from your local highway yard though. It may have other stuff in it. I talked to the folks at Yukon Tire and they have calcium chloride in 40 kilo bags for $60.00. 40 kilos will be enough to last you for oh say the next three or four hundred years. Not to worry, they told me that they sometimes have an open bag and if gardeners want it they will sell you a cup full which will only last you for the next decade or so.

Once you have the pH adjusted so the plants are able to use the Calcium and they get enough Calcium you shouldn't have a problem curing blossom end rot. It would also help if we could string together more sunny days in a row. Lets hope we get at least a couple for the Klondyke Harvest Fair coming up this weekend. Don't miss your chance to check it out!

For more information on curing blossom end rot check out:

  • New Eden
  • Kids Garden
  • Plant a Row Grow a Row