Fig. 1. Dutch and beit alpha cucumbers do not require pollination. They are parthenocarpic and form fruits without pollination.

What came first: the chicken or the egg? I cannot answer that question. However, in my experiences with hydroponic and greenhouse food crops, I can confidently tell you that before there are tomatoes, peppers, eggplants, strawberries — or any other type of fruit — there are flowers. Purposeful flower pollination is essential for most of our fruiting crops grown in controlled environment. While there are certainly many factors that affect fruit growth and development, pollination is paramount because it is the necessary precursor to fruit! This month's Hydroponic Production Primer will focus on pollinating food crops grown in greenhouses and controlled environments.

Pollination requirements

Before we discuss the pollination process and different pollination mechanisms, let’s review the pollination requirements of hydroponic crops. First, if you are producing leafy crops such as lettuces, herbs or other specialty leafy greens you do not need to worry about pollination; only foliage, not fruits, are harvested and sold.

Not all fruiting crops require pollination, either. Though field cucumbers require pollination, the types of cucumber used for hydroponic production such as the long Dutch or beit alpha types do not require pollination because these fruits are parthenocarpic; their fruits are formed without any pollination, resulting in seedless fruits (Fig. 1). Pepper can be grown without pollination, as they readily self-pollinate to form fruits. While bees may be used to increase the degree of pollination and to improve pepper fruit quality, they can also cause damage by visiting the flower too much and cause misshapen or unmarketable fruit if managed improperly.

The pollination process

All products of high-wire vining crops are fruits. In a culinary and (horti-)cultural sense we consider peppers, cucumbers and eggplants vegetables. In a botanical sense, they are fruits. Fruits are the product of a successful pollination. For the ovaries to grow and develop into fruits, viable pollen must be transferred from an anther to a receptive stigma. If the pollination is successful, the ovary enlarges into the different fruits that we harvest.

How do you know when a flower is ready for pollination? When a flower starts to open, it is not yet ready for pollination (Fig. 2). The pollen is not mature and dehiscing from anthers, and stigmas are not receptive to pollen. As the flower continues to open, the pollen and stigma mature. As pollen matures, you will see the anthers change their appearance to light greenish-yellow with a smooth appearance to a bright yellow surface that appears rough because pollen is starting to shed. Petals are another good indicator of pollination. When the flower petals are fully open the petals reflex back, and this is an easy indicator of a receptive flower.

In addition to having flowers which are physiologically ready for pollination, you also want to have a greenhouse environment that is conducive to pollination. Temperatures below the lower 60s or above the upper 80s (F°) are not conducive to pollen growth and subsequent pollination. Additionally, dry greenhouses are problematic for pollination and relative humidity should be maintained around 70 percent. You’ll especially want to monitor the humidity in your greenhouse in the winter, when heating can dry out the air, or in arid production environments.

Fig. 2. While the flower on this eggplant is fully opened, the pollen on the anthers is not dehiscing and isn’t ready for pollination.

Possible pollinators

Now that you know which crops require pollination and are familiar with the pollination process, let’s focus on how to approach pollination in your facility. There are number of different pollination methods, including by hand, with bees, or chemically.

Flowers can be pollinated by hand in a few different ways. First, pollination can be achieved by hand in several ways. It can be common to try and transfer pollen from the anther to the stigma by hand when pollinating a small number of plants, but this is a time-consuming task. Thankfully, if the environment is favorable and the pollen is mature the flower simply needs to be shaken to release a “puff” of pollen that will reach the stigmas and pollinate flowers. This can be achieved by hand-held vibrating tools placed on inflorescences. Another method I have seen is having someone walk up and down aisles, rapping twine trellises with a simple stick or plant stake; the vibration of the trellis moving can be sufficient in releasing pollen, but care must be taken not to physically damage plants. Regardless of which hand pollination method is used, it can be a time-consuming and expensive process

Bumblebees are the most common method used for pollinating greenhouse food crops, as they are an extremely effective and efficient method to pollinate large. Bumblebee hives can be ordered and shipped to your door. There are several reputable suppliers, and you’ll want to work to identify the one that works best for you; bumblebees are a perishable product that you will need to replace periodically. The idea of releasing a lot of bees in an indoor are with people may seem dangerous. However, taking a few precautions dramatically reduce the dangers. While bumblebees can sting, the chances of being stung are greatly reduced if you remain calm around the bees. Do not swat at them, and be mindful when handling plants to avoid grabbing a bee. Also exercise caution around the hives —you know what happens when you kick a hornet’s nest!

There is a chemical approach to pollinating some crops such as tomatoes. Auxin, the plant hormone most widely known for its use to promote root growth on cuttings, can be applied as a foliar spray to promote tomato flower pollination. However, this method is not widely used. Fruit quality from chemically pollinated flowers can be inferior compared to fruits from bee or hand-pollinated flowers. Additionally, this method can diminish the appeal of the product for some producers and consumers alike.

Fig. 3. Bumblebee hives are shipped out to growers and placed in the greenhouse, where they are effective and efficient pollinators.

Pollination problems

To have adequate pollination, start by having plants in the right balance of vegetative growth (i.e. leaves) and generative or reproductive growth (i.e. flowers). While not a pollination problem per se, the number of flowers that can be diminished if there is too much vegetative growth. Similarly, if plants are too reproductive, there may not be adequate vegetative growth to support the development of fruits from pollinated flowers.

Incomplete pollination is another problem with fruiting crops. When pollination is partial or incomplete, fruits do not fully develop and may develop lop-sided or uneven fruits as a result of few seeds. Seeds produce the plant hormone auxin, which promotes fruit development. Therefore, with fewer seeds, auxin levels are lower and fruits don’t grow to their full potential.

In addition to incomplete pollination, excessive pollination can reduce fruit quality. When flowers are visited by too many bees, the excessive bee activity can cause damage to the fruits. Peppers are a crop that can be adversely affected by excessive pollination; since they are already pollinated relatively easily without bees, using too many bees can diminish fruit quality.

The take-home message

For most fruiting crops grown in hydroponic systems, pollination is an integral step in producing your produce. By focusing on providing an environment conducive to successful pollination, steering crops to be appropriately balanced between vegetative and reproductive growth, and selecting the pollination method that works best for you, you can improve yields and consistency with your crops.

Christopher is an assistant professor of horticulture in the Department of Horticulture at Iowa State University. ccurrey@iastate.edu