Water deficits
As November started, many people were concerned that their fields needed water. Metro area weather forecasters regularly reminded their viewers that the metro area was 9 inches short of rainfall. They did not explain that being short 9 inches of rainfall does not mean that the metro needs 9 inches of precipitation to make up for the current water deficit. As winter starts, water deficits are seen in dry soil, rivers that barely flow and lakes with receding shorelines. Most soils will be fully recharged with about four inches of precipitation.
Dry soil can be a serious problem in the winter, and your soils stopped recharging after the soil froze. The news is better than commonly reported. Strawberries and blueberries have shallow roots, with 90% of the roots growing in the top foot. Sandy loam soils hold between 1.25 and 1.75 inches of water per foot, which means your fields will be recharged with 1.5 inches of rain. The official rain total for Minneapolis for early November before the soil froze was 1.61 inches of water. It would have been nice to get four inches of precipitation to fully recharge the soil, but there was enough precipitation across the state to keep your roots healthy this winter.
My biggest question
When I started my job in the fall of 2000, I asked all available experts if size in strawberry was primarily determined in the fall or spring. Nobody I asked had an answer. 22 years later, while reading scientific journals, I finally received my answer.
Growing large fruit and vegetables is more than bragging rights or funny pictures. Large fruit is easier to harvest, catches the attention of customers and means higher yields (in most cases). With onions, for example, there is a fixed number of plants. A 25% increase in average size means a 25% increase in yield and profit. Growing large apples and strawberries is far more complicated than annual plants like onions, but size is still critical.
There are cases where fruit is too big. My brother likes to say that the best quality belongs to the “right sized” fruit. For example, when growers started planting orchards in the desert of Eastern Washington, some of the Red Delicious apples were the size of small pumpkins. Those apples were soft and covered in bitter pit. Zestar by contrast is a large apple, but in some orchards about 5% of the apples are half the size of the normal apples. The small Zestar have poor quality. Strawberries are a notable exception to the “right size” rule. I rarely see poor quality on large strawberries.
Size in all fruit is partly determined by leaf area and leaf to fruit ratio. You can have lots of leaves without fruit, but you cannot have lots of fruit without leaves. All fruit crops need a specific number of leaves to properly size. For tree fruit, scientists have determined the amount of leaf area one fruit needs to properly develop. An apple needs a minimum of 15.5 square inches of nearby leaves (usually on a spur) for the apple to properly mature. If there are not enough leaves, the fruit will be smaller and usually poorer quality. Honeycrisp is especially susceptible to poor quality if over cropped. Tree fruit producers adjust the leaf to fruit ratio by thinning so that the there are fewer fruit for the existing leaves. Unthinned Honeycrisp taste terrible.
For stone fruit, the leaf to fruit area is the major factor determining fruit size. We spent a lot of money in Montana this last year thinning peaches. Thinning still could not make large fruit in the two blocks of certified organic peaches, which had small leaves due to a persistent nitrogen deficiency. Even after thinning, peaches in the organic block sized into tennis balls. We are slowly making progress on increasing vigor in the organic peaches, but it takes years to increase vigor in fruit trees and blueberries.
Strawberries are not amenable to fruit thinning. Instead, the first step to achieving sizable fruit is to ensure that there is adequate nitrogen to grow large leaves. Other nutrients are important, but P and K deficiencies are rare in this state. A strawberry field needs 70 pounds of nitrogen per acre to develop a healthy canopy. More nitrogen will not increase yield or fruit size. The strawberry growers I work with are so good at fertilizing that low nitrogen is uncommon.
Figure 2. Strawberries on the right were from plants that received no spring nitrogen. Note the reddish caps, which are a symptom of low N.
In the early 2000’s, many scientists who studied strawberries told growers to avoid fertilizing strawberries in the spring, because the extra nitrogen would cause excess growth, and excess growth increases gray mold and can even harm flavor. In 2004, one strawberry grower missed a few rows during his spring fertilization. I could see the N deficiency in smaller, light green leaves and reddish caps. The fruit on plants with low nitrogen was both smaller and tasted poorly. Since then, I have told all growers on sandy soil to use nitrogen in the spring.
Plant spacing. Agronomists dramatically increased corn yields in the 20th century by increasing the number of corn plants per acre and then increasing the amount of nitrogen to those plants. In fruit crops, increasing the plant population can decrease yields. In order to form fruit, plants need to form floral buds, and too much crowding can decrease the number of floral buds in plants as different as pumpkins, blueberries and strawberries.
When strawberry production started in the 18th century, gardeners realized that the highest size and yields were achieved at spacings of 8 inches. To achieve that spacing, a few runners were set by hand and the rest were cut off. Eventually, strawberry farms grew too large for growers to set or cut every runner. Appropriate spacing can be achieved with less labor by narrowing rows. Plants on the edge of a matted row receive the most sun and grow the largest fruit. Every so often a new grower will try maximize production by letting the entire field be covered in strawberry plants. Yields in those fields drops by as much as 90%. There is a tradeoff. The largest fruit is at 8 inch spacing, but higher plant densities can suppress weeds, which is why most of us go prefer a higher plant density. Some of the highest yielding farms have narrow rows that are placed closer than the standard 48 inches.
Seed number. Stone fruit are very simple: one ovary, one stile and one stigma. If the stigma isn’t pollinated, there is no fruit. Apples have five stigmas and five ovaries, and the flowers are pollinated by insects. If one ovary is not pollinated, the apple becomes lopsided. I see many apples with only one ovary that was pollinated, and the fruit is not commercially viable. Insect pollinators are critical to make sure that each seed capsule is pollinated.
Figure 3: the five stigmas and styles (top) of an apple blossom responsible for the five capsules of the apple fruit.
Strawberries have multiple ovaries and multiple stigmas, but the ovaries are on the surface of the fruit. Pollination isn’t that critical, because some pollination occurs when the flower is moved by wind. The key factor determining the size of a strawberry is the number of seeds. When trying to determine why a field has small fruit, I first look at the spacing of the seeds. If the seeds are close together, then lack of water or nitrogen could have contributed to the small size. If a small strawberry has fifteen seeds, there is nothing that grower could have done that spring to grow larger fruit.
Return to Figure 1. The seeds on the berries to the right are closer together than those on the left, showing that smaller size occurred because the fruit did not adequately fill.
Returning to the question. Floral buds are formed in the fall when the days shorten and the nights cool. Since the size of the fruit is determined by the number of seeds, I wanted to know if the seeds were formed in the fall or spring. If seeds are formed in the fall, then size would predominantly be determined in the fall. Even without the definitive answer, I started emphasizing management practices for the fall. Strawberries should be mowed early enough so that the plants have a full canopy before floral initiation in mid August. Initially, I told growers in northern Minnesota to mow before August 1, but later moved the date to July 24. I regularly emphasized that the most important fertilization of the year is during renovation, because healthy strawberry plants produce the largest flowers. Each fall, I estimated the potential yield of strawberry fields. Adverse weather can lower the yield between September and June, but fertilizer in the spring cannot turn a weak field in April into a highly productive two months later.
Finally, my big question was answered in an obscure scientific journal published last century, the results written in plain Japanese. No, I don’t read Japanese, but there was a translation of the abstract. The number of seeds is primarily (not exclusively) determined in the fall, and temperatures in the fall help determine the number of seeds. Strawberries don’t just need short days to develop floral buds, they also need cool temperatures. Flowers with the largest numbers of seeds are formed when the days are mild and the nights are cool during the floral induction period. In Minneapolis, the average October temperature this year was 51.8F. It turns out that average temperatures mean little to your berries, because strawberry floral buds will not form if the high is 54 and the low is 48. In Minneapolis, the average high in October was 62.1 while the average low was 41.5, which is almost ideal.
Strawberries are incredibly complicated, even by the difficult standards of fruit science. Strawberry fruit formation follows the same principles as stone fruit and apples, but with additional layers of complexity that I struggle to understand. This year, conditions were good during the fall, and many of you could have a great crop next summer. In the meantime, relax and try to enjoy the winter.