After a story of mine about too-early planting of tomatoes appeared in the Chicago Tribune, a newbie gardener who had suffered a lot of blossom end rot in her first year of container tomato growing asked me how she could avoid it this year. Here's what I told her:
Blossom end rot in container tomatoes is usually related to watering, which affects the plants' ability to take up calcium. Most good-quality commercial potting mixes have sufficient calcium. If you want to be extra sure, mix a handful of bone meal (which is about 15 percent usable calcium) or 1 tablespoon of dolomitic limestone in the potting mix before you plant. Mix it in well with all the soil in the container before planting and don't use too much.
Soil pH also affects the plants' ability to take up calcium, but again, a good-quality potting mix should have a reasonable pH range. So don't sweat that.
It is much more likely that the rot comes from over- or under-watering. Tomatoes need a steady, even supply of moisture, not swinging from wet to dry and back again. If you are using self-watering containers, make sure that the reservoir has an overflow that is not clogged so that surplus water can drain away.
Use a good-quality, fluffy, well-draining potting mix high in organic matter that will wick up moisture from the reservoir without becoming saturated and soggy. Mulch over the surface (I use cotton burr compost, but any fine organic mulch will do) to reduce evaporation. The goal is to keep the soil steadily just moist enough for the roots to absorb the water (and calcium) they need.
Other factors that could be involved:
-- Planting too soon. Planting tomatoes into cold, wet soil leads to all kinds of problems and can contribute to blossom end rot. Even though soil in containers warms up faster than in the garden, it's best to wait until the weather has truly warmed up. The last couple of weeks it has been 10 or 15 degrees colder than normal. Way too cold to plant tomatoes.
-- Sticky, poorly draining clay soil. This could be a problem if you tried to use garden soil in your containers, or if you bought something labeled "potting soil" rather than "potting mix." "Potting soil" is generally not much more than construction debris. It is usually heavy, sticky, and full of rocks and junk, with little organic matter. That's why it's so cheap. "Potting mix" is light and has lots of organic matter. It costs more and it's totally worth it. Some gardeners mix their own potting mix, but if you're growing in containers, it's much easier to buy a good-quality mix in bags.
-- Overfertilizing. Too much fertilizer can upset the balance between calcium and other minerals in the soil. It's best to use a slow-release fertilizer mixed in the soil when you plant (some mixes come with fertilizer mixed in), and add another handful of slow-release on the surface of the soil in early July, as the plants are starting to set fruit. If you use water-soluble fertilizers every time you water, it's too tempting to use too much. I use an organic fertilizer such as Espoma Tomato-Tone or a fish and seaweed emulsion such as Neptune's Harvest. These organic fertilizers are much more gentle and less likely to throw your soil out of whack.
Make sure you use a fertilizer in which the three numbers in the nitrogen-phosphorus-potassium ratio are balanced (such as 10-10-10 or 5-5-5), or the middle number (phosphorus, which helps with flowering and fruiting) is higher. That's what you want for fruiting vegetables such as tomatoes. Avoid fertilizers in which the first number (nitrogen) is high; that pushes foliage at the expense of fruit.
-- The right plant. If you use a huge, sprawling, indeterminate tomato in a container, you might find yourself pruning and staking it a lot. Heavy pruning can give the plant problems adjusting its water needs. In my containers, I use cages to contain the plants, rather than stakes, and start with a "dwarf" or "bush" or "container" or "patio" variety of tomato that is suitable for containers. Most are determinate varieties.
-- Attention. Pay attention to the overflow gauge of your self-watering containers; fill when you need to and not when you don't. Remember that rain will soak through the soil and down into the reservoir. But even with self-watering containers, trust your own senses more than the gauge. Regularly stick your finger into the soil to make sure it's moist, but not soaked or dry. Tomatoes are not set-it-and-forget-it.
I'll be giving a talk called "Vegetables Anywhere: How to Fit Farming into Your Life" at 9:30 June 2 at the Prestwick Area Garden Club, which meets at the Prestwick Country Club, 601 Prestwick Drive, Frankfort (info: 708-917-8033). It's about small-space vegetable gardening, including container gardening. I've gotten good reviews after giving this talk at the Irving Park and Wicker Park garden clubs recently.
Got a garden question? I recommend you call or e-mail the Plant Clinic of The Morton Arboretum in Lisle, the Master Gardeners of the University of Illinois Extension or the Plant Information Service of the Chicago Botanic Garden in Glencoe .
All contents of this post are copyright Beth Botts. Feel free to link or share a brief excerpt with a link, but please do not reproduce photos or any other part of this blog without my express permission.
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