Nectar plants explained

What nectar is

Nectar is a sugar solution secreted by a nectary, usually at the base of a flower. It is typically 15 to 75 per cent sugar — mostly sucrose, glucose and fructose — with traces of amino acids, minerals and aromatic compounds. Its function is to reward a visitor for carrying pollen.

It is an expensive thing to make. A plant producing nectar is spending carbon it could have used elsewhere, which is why nectar production is tightly regulated and why flowers stop producing once pollinated.

How plants control the transaction

• Concentration. Higher sugar concentration is more attractive and more viscous, which is why bird-pollinated flowers are dilute and bee flowers more concentrated.
• Volume and refill rate. Viper's bugloss refills within minutes, which keeps a bee working the same plant. Others offer one dose and are done.
• Corolla depth. A deep tube excludes short tongues entirely, restricting access to a preferred pollinator.
• Timing. Some flowers secrete only in the morning, or only at night for moths and bats.
• Guarding. Some flowers close, or hide the nectar behind a structure that must be operated.

Nectar guides and access

Many flowers advertise where the nectar is, using patterns visible in ultraviolet that a bee can see and we cannot. A plain yellow flower may carry an unmistakable bullseye to a bee.

Access is the other half. A honey bee's tongue is about 6 mm, a garden bumblebee's about 15 mm, and red clover's corolla is around 9 to 10 mm deep. That single arithmetic explains why red clover is a bumblebee crop, why New Zealand imported bumblebees in the 1880s to pollinate it, and why short-tongued species bite through the base and rob it instead.

Extrafloral nectaries

Some plants produce nectar outside the flower entirely — on leaf stalks, stipules or bracts. Cotton, sesame, cherry and many legumes do this. The function appears to be defensive: the nectar attracts ants and parasitic wasps, which then attack caterpillars on the plant.

For beekeeping this is a bonus. Extrafloral nectaries secrete over a longer period than the flowers, which extends the useful forage window of a crop well beyond its bloom.

What makes a good nectar plant