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Flower stalks put an end to some garden goodies in the summer heat

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Flower stalks put an end to some garden goodies in the summer heat
Wadena Minnesota 314 S. Jefferson 56482

The heat of the summer is upon us and the ascent of a flower stalk puts a bitter end to a sweet crop of lettuce and/or radishes. So what happens in these vegetables so that they can not be enjoyed all season? The obvious answer is that they turn bitter or woody and bolt. So what causes this?


In lettuce the chemical which is responsible for the bitter taste is sesquiterpene lactose. This chemical is always present in the lettuce plant. It combines with the sugars in the lettuce plant, but this chemical increases within the plant as it begins to mature or bolt and form a seed head.

The bitter taste and bolting of lettuce is not just brought about by the heat. In 1995, Dr. William Waycott, a plant geneticist, did research for the United States Department of Agriculture in Salinas, Calif., on the growing of lettuce. He grew lettuce in growth chambers under short eight hour days of sunlight at a steady temperature of 90 degrees. These plants grew to maturity without bolting.

He continued to follow these plants (short eight hour days), and after 135 days these plants begin to bolt. Other lettuce plants grown under normal condition, or what is called long days of sunlight, bolted at 90 days. This can be explained by the existence of a genetically controlled clock like mechanism within the lettuce that tallies the number of light hours required for a given cultivar to bolt.

Nevertheless there are indications that in addition to longer days, high temperatures can stimulate bolting in the late stages of leaf growth. It also may be caused by drought conditions or lack of moisture which often occurs with high temperatures.

Bigger isn't better among radishes

The biggest radishes are not necessarily the best radishes. Radishes are best when they are the size of a large marble. Radishes bolt and go to seed in the summer heat. However, it is believed that the length of days here again rather than the temperature may influence the bolting. When grown in controlled conditions where they received sun for only eight hours and were then covered, radishes did not bolt in the heat.

When growing radishes in your garden they need to grow quickly to ensure they are tender and plump. If they don't get cool temperatures and lots of regular water, their growth will slow (uneven watering causes them to split) and they will become hot, tough, woody and dry. When the temperatures start to warm up, it encourages the growth of the radish tops to bolt and begin to form their seed heads. The growth on the top causes the growth of the radish bulb to slow or stop. Radishes planted too thickly (they need 1 to 2 inches between plants) will feel they have no room to plump and will then go to seed.

The radishes we grow in our gardens are spring radishes and they should mature in 20 to 30 days. Spring radishes do not keep as well as the winter type radishes that we buy in a supermarket. Winter radishes take 50 to 60 days to mature and have a better storage life.

Later in the summer is a good time to replant lettuce and radishes so that we can again enjoy them as the cooler temperatures favor their growing conditions.

Why are cucumbers bitter?

Another great vegetable that is coming on now is the cucumber. However, sometimes cucumbers too have a taste bitter. This is the result of cucurbitacin, a compound usually found in the stems, roots, and leaves of the plant. Occasionally it spreads into the fruit. When this happens it is concentrated more at the stem end of the cucumber.

Researchers aren't entirely sure what causes some fruit to be more bitter than others. It is thought that stress from drought or lack of water and cool growing seasons may be the factor. For tasty cucumbers, keep the plants watered, feed them regularly and grow varieties which are less prone to bitterness. According to the Washington State University, such cultivars are Ashley, Improved Long Green and Lemon.

Kyle Schulz is a Wadena County Master Gardener from Sebeka, and the regular gardening columnist for the Wadena Pioneer Journal.