Written by Micah Kelleher, with guidance from Collin Luebbert, for use in the Baxter Lab at Donald Danforth Plant Science Center.
A typical day of maize pollination and preparation involves three main tasks: shoot bagging, pollinating, and setting up pollinations for the following day.
- Mesh aprons – Midco Mesh Aprons
- Stapler with staples – Ace Clipper
- Tassel bags – Brown Tassel Bags
- Shoot bags – Pollination Bags Shoot
- Shears
Maize (Zea mays) is a monoecious plant, meaning that each plant contains both male and female reproductive structures. However, these organs are in imperfect, or unisexual, flowers. Male and female inflorescences are separate, but both are found on the same plant.
The male flowers are located in the tassel at the top of the plant. Tassels produce pollen, which is released from structures called anthers. Each tassel consists of many small units called spikelets, each containing two florets enclosed by glumes. Inside the florets are three anthers, attached to fine filaments. As the florets mature, the anthers emerge and dangle from the tassel, releasing pollen. This can be seen as a yellowish dust when the tassel is shaken.
The female flowers grow lower on the plant and develop into ears. These arise from the leaf axils, which are the upper areas between the leaf sheath and stalk. Each ear contains many ovules, and each ovule produces a silk that serves as both the stigma and style. The silk captures pollen and transports it down to the ovule for fertilization. One successfully pollinated silk results in one kernel on the cob.
Figures 1 A–C:
- (A) Ear before pollination with silks emerged and attached to the ovules.
- (B) Tassel shedding pollen.
- (C) Tassel no longer shedding pollen, with spent anthers.
Shoot bagging can be done at any time during the day, but it is the first step in controlled pollination. It can be done before pollinating while waiting for moisture to dry from the bags or after the morning's pollinations are complete. Shoot bags, made of waxy paper, are placed over shoots on plants that will receive pollen to control which pollen contacts the silks. The longer flap, or tongue, should be positioned against the stalk. To secure the bag and prevent it from being dislodged by wind, pull it downward firmly. We typically focus on the top three uncontaminated ears per plant. If silks have already emerged from a shoot, it is considered contaminated and should not be bagged.
Figures 2 A–C:
- (A) Shoots before bagging.
- (B) Shoots after bagging.
- (C) Shoots with exposed silks, which is considered contaminated.
Pollination is performed in the morning using the tassel bags and shoots that were set up the previous day. We typically pollinate between 8:30 AM and 11:00 AM. This allows time for moisture on the bags to evaporate and avoids the high heat later in the day (especially above 95°F). If nighttime lows reach 55°F or lower, viable pollen may still be available, but it’s best to wait until a few hours after temps are above 55°F for better shedding conditions. High humidity can also hinder pollen shedding. It should also be noted that pollen submerged in water is no longer viable, so moisture on the tassel and shoot bags should be avoided.
Pollen generally remains viable for about 20 minutes after leaving the anther. In hot, dry conditions this can be reduced to 10 minutes or less. In cool, humid weather, it may survive 30 minutes to several hours. For this reason, we bag tassels the day before, ensuring that any unwanted pollen on the tassel will no longer be viable the next morning.
To collect pollen, bend the plant so that the tassel bag’s opening is horizontal or slightly upward to prevent pollen from spilling out. Tap and shake the bag gently to release the pollen, remove the staple and the bag, then immediately walk it to the receiving plant.
You may lean the plant receiving the pollen to avoid accidental contamination by stray pollen. Remove the shoot bag from the ear and pour the pollen directly onto the exposed silks. You can channel the pollen using a crease or corner of the tassel bag for better control. It should be noted that the pollen, which is dust-like, should be poured onto the silks, not the anthers alone. If using the pollen on multiple plants, one bag of pollen can typically be used for two to three plants.
Figures 3 A–C:
- (A) Ear before pollination with silks emerged.
- (B) Ear being pollinated with anthers contacting silks. Anthers can make contact with the silks, but they will not ensure pollination.
- (C) Ear being pollinated without anthers contacting silks. Pollen can be poured onto the silks without including anthers, which can help ensure contact.
After pollination, cover the ear with a labeled tassel bag, wrap it around the stalk, and secure it with staples. Pre-labeling the tassel bag is ideal, but if more time is needed, the shoot bag can be placed back over the ear. However, this may not be ideal if the shoot bag is very wet. You may reuse the tassel bag with pollen to cover an ear once pollinated. But when pollinating multiple plants from one bag of pollen, you will need to cover some ears with new tassel bags. These bags should be labeled with the female source (which can be row, plot, or genotype depending on your nursery's arrangement) x male source (e.g. 7 x 10). For a self, write the female source and draw a circle with an "X" through it (e.g. 7 ⓧ). Always include the date and source(s) on the bag.
Figures 4 A–C:
- (A) Tassel bag bent over before shaking and removing pollen.
- (B) Tassel bag stapled over the ear.
- (C) Labeled tassel bag on for a cross-pollinated ear formatted as (female plot x male plot). If a tassel bag with a calendar is used, a mark is drawn through the date. Otherwise, the format is Month/Day. In the case where each cross is recieving pollen from the same parent line, you can use use the format plot x.
To set up for the next day’s pollination, check that the tassels on donor plants are shedding and that the recipient shoots have silks. We usually focus on the highest uncontaminated ear. Only one ear is typically pollinated per plant to maximize kernel development.
Remove the shoot bag of the recipient. Before cutting, feel for the location of the ear to avoid cutting into it. Using shears, trim the top of the shoot to expose the silks. This maximizes the number of silks that will be able to receive pollen. If no silks are visible after trimming, the ear is likely not ready or wasn’t trimmed down enough.
If the shoot is trimmed and silks are present, replace the shoot bag immediately and fold the top edge outward and away from the stalk. Mark the fold with an "X" if it was set up on an even date and an "O" if on an odd date. This helps pollinators easily identify set up ears. It may also be helpful to keep track of how many plants are set up in each plot and communicate that to the next day's team.
Figures 5 A–C:
- (A) Shoot cut back properly with silks exposed.
- (B) Shoot cut back with no silks visible. In this case it was cut prematurely.
- (C) Shoot cut too far, slightly into the cob. It can still be pollinated, but not ideal.
Next, find a pollen donor plant that is shedding pollen and cover the tassel with a labeled tassel bag. Include the date and, if the male parent is not assumed, the male parent source. Any pollen present at the time of bagging will not be viable the next day, and only newly shed pollen will be viable when pollinating. Fold the lower part of the bag in half along the stalk and fold up the corner to form a triangle against the stalk. Use one staple to hold in place.
Figure 6:
- Labeled tassel bag stapled around the tassel.