Guest Author - Jessica Carson

Most gardeners who have grown tomatoes have seen their crop damaged by blossom end rot at least once, and would like to prevent it from ever happening again. The start of blossom end rot looks like dark, wet mushy spots on the fruit where the bloom has dropped off, and can appear at any time in the fruit's development. The tomato lesion will eventually blacken and harden, and may crack, where on peppers, eggplant, squash and melons the lesion may appear brown. Blossom end rot may appear as a small spot, or can enlarge to cover 1/3 to ½ of the fruit. Secondary infections of the fruit may occur, such as molds or insect invasions. Fortunately, blossom end rot is fairly easy to prevent by taking a few simple steps, once the causes are understood.

Basically, the cause of blossom end rot it not enough calcium in the plant to meet its growth and fruit development needs. This can be caused by several factors: the soil is too cold for calcium uptake, large fluctuations in soil moisture (too dry to too wet to too dry again) prevent the plant from absorbing enough calcium, damage to the tomato root system preventing adequate calcium uptake, too many ammonium or other competing ions in the soil, and not enough available calcium.

Cold Soil
If tomatoes are planted too early in the season, the container soil may be too cold for the plant to absorb adequate amounts of minerals, including calcium. In this case, the first fruits may show signs of blossom end rot, but fruits set when the weather has warmed do not. One solution is to move your containers where they will receive more direct sun and warmth during the day, then protect them from cold at night.

Fluctuations in Soil Moisture
If the soil is allowed to dry out completely between waterings, then the container is drenched, then the soil dries out again, the root system will be unable to absorb enough calcium for the needs of growth and fruit development. Plant your tomatoes in a rich, well-drained soil, and water whenever it is dry at 1' to 2” deep. Do not keep the soil constantly wet, but don't ever let it completely dry out, either.

Damage to the Root System
If you buy a large tomato plant from the nursery which already has set tomatoes, be careful when transplanting to not damage the roots. Damage to the roots may disrupt the calcium uptake for a while, and your first fruits may suffer from blossom end rot. Also, be careful when digging around your established plants. If you wish to grow several things in one large container, be careful not to disturb the tomato roots while you plant.

Note that tomato plants are heavy rooters – requiring a lot of space for their expensive root systems. I've always had the best success by planting one tomato plant per large container.

Too much Ammonium
If you wish to use a prepared chemical fertilizer, use a nitrate nitrogen fertilizer as the nitrogen source. Ammoniacal nitrogen may increase blossom-end rot as excess ammonium ions compete for the calcium on the root receptor sites, reducing calcium uptake. Also, use fertilizers that are not too heavy in nitrogen content. High nitrogen will cause the plant to leaf out rapidly and the available calcium will flow primarily to the leaves, leaving blossom end rot as the result.

Not Enough Calcium in the Soil
Knowing that tomatoes have a large calcium need, it's best to give them extra when you plant. Crushed egg shells are an excellent source of calcium. Save your eggshells and dry them out, then pulverize them by hand, in a food chopper, or with a coffee grinder. Add a handful to the hole when you transplant your seedlings, or mix in a few handfuls when preparing your soil if you are direct seeding. If you are using a self-watering container, add a layer of crushed eggshells near the bottom of the soil so your plants will absorb the calcium as they take in the water. If you still see signs of blossom end rot, mix crushed eggshells, crushed limestone (lime), or crushed gypsum into some soil mix and side dress your tomato plants. Tomatoes love to be planted deeply, anyway, and will grow additional roots along the submerged stem, so feel free to heap the calcium-rich soil up around your plants.