The rain dance worked but my tomatoes are rotten

Published 4:06 pm Thursday, June 27, 2024

Getting your Trinity Audio player ready...

We got some rain this week! Hopefully, you saw my article on drought last week. I want to continue that discussion with a couple of other problems that have become visible this week. First is herbicide application. I received a question this week about why an herbicide application had no effect.

Remember, herbicides are used to kill undesirable plants. Some folks call them weeds but others, like me, describe them as a plant out of place. There are selective and non-selective herbicides that you can purchase. Selective herbicides can be applied on or around desirable plants to kill undesirable plants but do not harm the plant we want to keep. These are most often used around trees, shrubs, lawns, and vegetables. An example would be using a herbicide formulation with the active ingredient sethoxydim to kill crabgrass around your vegetable. This product will kill most grasses without harming most broadleaf plants. Non-selective herbicides are used to kill almost any plant. Formulations with the active ingredient glyphosate (Roundup) will not be selective in what it kills.

We can classify these herbicides in another way, systemic or contact herbicides. Both examples are systemic herbicides meaning they are adsorbed into and translocated within the plant. Systemic herbicides tend to provide a more lasting result because they kill the roots and the foliage. Contact herbicides are not adsorbed into the plant, as such they will kill the parts they touch and often will do nothing to the root. Contact herbicides will show results much quicker, often only taking hours to see the plant’s response. Systemic herbicides can take up to three weeks.

To answer this caller’s question requires investigation. There are a number of reasons why a herbicide doesn’t work. First, did they mix and apply the herbicide correctly? Using a lesser rate than what is on the label can result in not only not killing the plant you’re after but can also contribute to herbicide resistant plants in the future.

Always read and follow the label, including any Personal Protective Equipment (PPE). Check your measurements twice when mixing. If a systemic herbicide was used, remember, it could take up to three weeks to kill the undesirable plants. This result can be compounded by lack of rain, cold temperatures, or cloudy days.

This is a good segue way to reason three why the undesirable plants didn’t die, a lack of water. If plants are stressed or otherwise not growing well, the effect of a herbicide will be greatly reduced, so much so that they may not work at all. A plant that is growing well will not die well. For example, there is only a two and a half to three-month period when Bermudagrass can effectively be killed, this is during the summer when it is actively growing. Other times we can slow it down, but it isn’t going to die.

Okay, enough about herbicides, what about this tomato issue? Some areas of Beaufort County received upwards of an inch of rain Monday and other areas saw none. We are dry, even in the areas that received the inch. Sporadic irrigation or rain on tomatoes is a contributing factor in the physiological issue of blossom end rot. The problem is associated with a calcium deficiency in the fruit.

There are several reasons why we could see this calcium deficiency response in tomatoes. Irregular or sporadic irrigation/precipitation is one cause. When it is dry, plants can’t take up much water which can lead to deficiencies. Another reason is when gardeners plant early into cold soils. Tomatoes need warmth to drive transpiration; the main way plants draw soil water solution into the roots and leaves. So, if it is cold, often the plants will show deficiency symptoms. A lack of calcium in the soil is possible but rarely the case for blossom end rot.

Most often, it is because the pH of the soil is too acidic. Tomatoes do best in a soil pH of 6.5. When the pH is on the more acidic side of the scale (<6.0), calcium is too attracted to the soil particles and becomes unavailable to the plant. Remember, plants can only take up nutrients that are dissolved in soil solution. If the calcium is too strongly attracted to the soil particles it can’t dissolve in the solution. If irrigation/precipitation is the reason for your problems, you will also see cracking in the shoulders of your tomatoes. Irrigating the plants on a schedule is very helpful as well as mulching. A weed-free straw is good to use for garden mulch. The straw will break down over the season and can be tilled into the soil to build it once the season is over. Mulch helps regulate soil temperature and moisture lessening the impact of irregular irrigation/precipitation. Mulch will also help to keep the weed pressure down. I would always suggest taking a soil sample and sending to NCDA&CS for testing prior to planting. Soils can be amended before planning but amending is much more difficult once plants are in the ground. Amending soil with lime can take several months to raise the pH, so it is best to take your sample in September or October. This allows ample time to receive the results and amend the soil in the fall so it is ready to plant in the spring. If the pH is low, NCDA&CS will recommend applying lime to bring the pH into range. Adding agricultural gypsum can help if the report shows that the pH is within range, but calcium is low. This can raise the amount of “free calcium” without raising the pH. If you have plants in the ground and did not amend the soil according to a soil report, I suggest a few strategies. First, there are products (End Rot, Stop Rot) that can be purchased to apply to the foliage and fruit that contain calcium chloride. This is only scratching the surface because plants do not take in much of the calcium through the foliage and the waxy covering of the fruit can prevent the fruit from taking in the calcium. These products can be applied to the soil as a drench for uptake. If you are having an issue in your home garden or landscape, send your questions to Gene Fox, Consumer Horticulture Agent with the North Carolina Cooperative Extension Service, please email at Gene at or call at (252)946-0111. You can also ask to speak to a Master Gardener! On Mondays and Wednesdays between 10:00 and 12:00, they are on the Greenline to answer all your home horticulture questions. Check out our Facebook page, Beaufort County Master Gardeners, for The Plant of the Week and Food Garden Friday! Until then, Happy Gardening!