For Plants, Everyday is “Sun” Day!

In the northern hemisphere the summer solstice marks the moment when we enjoy the most hours of sunlight per day. From here on out until the Winter solstice that falls on December 21st, 2021, every day in the Northern hemisphere will have just a little bit less sunlight than the day before. This matters to plants and to the gardeners that love and care for them.

Plants aren’t as dumb as some people think. Many plants, like some of the interesting rainbow colored carrot varieties, will very happily sprout and grow vigorously no matter what time of the year they’re planted- but they won’t form a fat, carrot-like root. But if you plant these day length sensitive varieties after the solstice these carrots will notice that the days are getting shorter. They understand that winter is coming and they’ll form nice, fat, sugar-packed roots so that they have enough stored energy to power the growth of a flower stalk in the following spring. As a consumer, I enjoy purple, red, and even “black” carrots, but as a farmer I know to respect the sensitivities of these kinds of carrots and I wait until after the solstice to plant them.

Radishes are another crop that can be very aware of day length. True, the supermarket varieties, like basic “red radishes,” will form a round little spicy globe-shaped root no matter the season of year. But many of the larger, so-called “storage roots,” like Watermelon radishes, Black Spanish radishes, or the Green and Purple daikon types, will only form a fat root if planted at such a time so that the majority of the plant’s growth comes during the declining days after the solstice. The very largest Watermelon radishes are produced by planting right around the summer solstice so that the plant frames up during the longest days and can capture the most energy and make the biggest root. Watermelon radishes planted late in summer or early in fall, will not enjoy as much of the sun’s energy, so they’ll form a smaller root. If you plant these radishes in the dead of winter they’ll sprout alright, but under the influence of the ever longer days after the Winter solstice they’ll “run straight to stick,” as we say, and make a flower stalk without ever forming a root.

Onions are very light sensitive. There are many varieties of onion that are adapted to grow at different latitudes. We’re on the 38th parallel, which means our days get longer than they ever will closer to the equator. The “Maui onion” is justly famous for its sweetness, but as Maui lies along the 20th parallel and we’re at the 38th, they just don’t perform well here. Success with growing onions starts with understanding and respecting each onion variety’s relationship with the sun. Father’s Day comes once a year, but for plants every day is Sunday.