Craig Sheaffer, Dept. Agronomy and Plant Genetics; and Deborah Samac and Mindy Dornbusch USDA-ARS, Plant Science Research Unit
Alfalfa’s value as a nutritious livestock feed, as a contributor of N to crops in rotations, and as continuous living cover is well known. It is an economically and environmentally valuable crop. But often overlooked is the simple beauty of alfalfa’s lush green foliage flowing over the landscape and the accent provided by its deep purple flowers. A closer look at the foliage reveals dainty leaves composed of trifoliolate leaflets and small flowers grouped together in a cluster called a raceme.
Individual flowers have a large standard or banner, 2 wings, and a fused keel petal. Within the keel petal are the male and female reproductive parts that are only obvious when the flowers are sprung or tripped by insects or wind.
Purple is the dominant flower color of modern alfalfa varieties; however, alfalfa varieties can have plants with a diversity of other flowers colors including blue, yellow, cream and white. Some flowers can have mixed colors (called variegation). Intensity of flower color varies considerably and can be affected by growing conditions and flower age. For example, purple pigmentation varies from a very deep purple to a light purple. Similarly, yellow pigmentation can vary from dark orange-yellow to very light yellow. Flower color is genetically controlled and due to expression of pigments such as anthocyanins in purple flowers or xanthophyll and carotene in yellow flowers. Some plants have flowers with vein pigmentation. The diversity of flower colors within some varieties is in large part due to the intermating of the purple flowered Medicago sativa with the yellow flowered Medicago falcata, a related subspecies of M. sativa. A non-pigmented, white flower is produced by recessive genes. A system for classification of over 20 alfalfa flower colors was established by USDA-ARS researcher, Don Barnes. For more information, visit https://www.naaic.org/Resources/colorguide/flowercolor.html.
Flower developmental stages have been widely accepted as an approach to scheduling alfalfa harvest. Forage yield and forage quality are affected by alfalfa maturity at harvest and harvest timing recommendations are often based on a target maturity stage (Figure 1). For example, the stage to cut alfalfa for high nutritive value is about the early bud stage. More mature alfalfa that provides greater yield will have lower nutrient content.
Alfalfa is a long-day plant that flowers when days are long ( ~12-15 hours), but flowering can be delayed by cool spring weather. Therefore, to remain on a seasonal schedule, alfalfa is sometimes cut by canopy height or calendar date. Also, in the fall, when daylength is decreasing and flowering is limited, harvests are often by date.
The appearance of flowers is also a good indicator of the status of energy reserves in the roots and plant persistence. Persistence will be greatest if harvest occurs at flowering stages because root reserves are maximized at full flowering. After plants are about 6 inches tall, they will start to recharge root and crown carbohydrate reserves for the next regrowth cycle. Multiple harvests at bud stages will reduce stand persistence.
A.Fernandez et al., 2019. Alfalfa, wildlife and the environment. North American Alfalfa Improvement Conference. https://www.gruppocarli.com/content/uploads/2024/04/Alfalfa-environment-2-1.pdf
V. Morell, 2017. Flower petals have ‘blue halos' to attract bees. https://www.science.org/content/article/flower-petals-have-blue-halos-attract-bees#:~:text=The%20size%20and%20spacing%20of,UV%20parts%20of%20the%20spectrum.
North American Alfalfa Improvement Conference. 2024. Description of alfalfa cultivars and germplasm. https://naaic.org/resource/cultivar.php
C. Sheaffer, N. Drewitz and J. Goplen. 2024. Managing risk at the first alfalfa harvest.
https://blog-crop-news.extension.umn.edu/2022/05/managing-risk-at-first-alfalfa-harvest.html
C. Sheaffer. 2024. Perennial forages provide continuous living cover and so much more! https://blog-crop-news.extension.umn.edu/2024/01/perennial-forages-provide-continuous.html
D.J. Undersander et al., 2014. Alfalfa management guide. https://www.agronomy.org/files/publications/alfalfa-management-guide.pdf
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| Alfalfa at flowering stage |
Individual flowers have a large standard or banner, 2 wings, and a fused keel petal. Within the keel petal are the male and female reproductive parts that are only obvious when the flowers are sprung or tripped by insects or wind.
Purple is the dominant flower color of modern alfalfa varieties; however, alfalfa varieties can have plants with a diversity of other flowers colors including blue, yellow, cream and white. Some flowers can have mixed colors (called variegation). Intensity of flower color varies considerably and can be affected by growing conditions and flower age. For example, purple pigmentation varies from a very deep purple to a light purple. Similarly, yellow pigmentation can vary from dark orange-yellow to very light yellow. Flower color is genetically controlled and due to expression of pigments such as anthocyanins in purple flowers or xanthophyll and carotene in yellow flowers. Some plants have flowers with vein pigmentation. The diversity of flower colors within some varieties is in large part due to the intermating of the purple flowered Medicago sativa with the yellow flowered Medicago falcata, a related subspecies of M. sativa. A non-pigmented, white flower is produced by recessive genes. A system for classification of over 20 alfalfa flower colors was established by USDA-ARS researcher, Don Barnes. For more information, visit https://www.naaic.org/Resources/colorguide/flowercolor.html.
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| Plants of most modern alfalfa varieties have purple flowers, but within a variety plants can have an array of flower colors and hues. Dark yellow flowers are typical of Medicago falcata, a subspecies of the purple flower Medicago sativa. Variegated flowers contain a mix of colors. Photos: Craig Sheaffer |
What flowers tell us
Variety characteristics
Although modern alfalfa varieties often have purple flowers, they often have a small percentage of other flower colors. This is because an alfalfa variety is a population of genetically diverse individual plants and not made up of genetically similar plants like a corn hybrid. While alfalfa flower color variation can be used by plant breeders to partially understand the genetic background of a variety, varieties should be selected by producers based on fall dormancy, winterhardiness, insect and disease resistance regardless of flower color. Variety descriptions are available at https://www.alfalfa.org/pdf/2023_Alfalfa_Variety_Leaflet.pdfHarvest scheduling
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| Figure 1. The relationship between alfalfa maturity stage and total forage, stem, and leaf yield and forage digestibility. Source: C. Sheaffer. In Undersander et al., 2014. Alfalfa Management Guide |
Alfalfa is a long-day plant that flowers when days are long ( ~12-15 hours), but flowering can be delayed by cool spring weather. Therefore, to remain on a seasonal schedule, alfalfa is sometimes cut by canopy height or calendar date. Also, in the fall, when daylength is decreasing and flowering is limited, harvests are often by date.
The appearance of flowers is also a good indicator of the status of energy reserves in the roots and plant persistence. Persistence will be greatest if harvest occurs at flowering stages because root reserves are maximized at full flowering. After plants are about 6 inches tall, they will start to recharge root and crown carbohydrate reserves for the next regrowth cycle. Multiple harvests at bud stages will reduce stand persistence.
Pollinators
Flower color is important in attracting a diversity of insects who use alfalfa nectar as a food source. These insects are part of the ecosystem food chain. Alfalfa nectar can be used for honey production by the honeybee although alfalfa is not a preferred source of nectar for honeybees. Insect attraction to flowers is due to a combination of factors such as smell, nectar content, and flower color. Although bee preference among alfalfa flower colors is unknown, some reports indicate that wild bees are attracted to flowers in the violet-blue color range which includes the purple flowers of most alfalfa cultivars. Availability of nectar from alfalfa is maximized by allowing plants to reach full flowering so delaying alfalfa harvests will increase insect visits and potential honey production.Additional sources of information
D. K. Barnes. 1972. A system for visually classifying alfalfa flower color. Agriculture Handbook No. 424. USDA ARS https://www.naaic.org/Resources/colorguide/flowercolor.htmlA.Fernandez et al., 2019. Alfalfa, wildlife and the environment. North American Alfalfa Improvement Conference. https://www.gruppocarli.com/content/uploads/2024/04/Alfalfa-environment-2-1.pdf
V. Morell, 2017. Flower petals have ‘blue halos' to attract bees. https://www.science.org/content/article/flower-petals-have-blue-halos-attract-bees#:~:text=The%20size%20and%20spacing%20of,UV%20parts%20of%20the%20spectrum.
North American Alfalfa Improvement Conference. 2024. Description of alfalfa cultivars and germplasm. https://naaic.org/resource/cultivar.php
C. Sheaffer, N. Drewitz and J. Goplen. 2024. Managing risk at the first alfalfa harvest.
https://blog-crop-news.extension.umn.edu/2022/05/managing-risk-at-first-alfalfa-harvest.html
C. Sheaffer. 2024. Perennial forages provide continuous living cover and so much more! https://blog-crop-news.extension.umn.edu/2024/01/perennial-forages-provide-continuous.html
D.J. Undersander et al., 2014. Alfalfa management guide. https://www.agronomy.org/files/publications/alfalfa-management-guide.pdf
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