Why Cosmos bipinnatus (Mexican Aster) Is Perfect For the Bee Garden


Cosmos bipinnatus, also commonly known simply as 'Cosmos', Garden Cosmos, or Mexican Aster is a stunning flower that will not only provide you with cheerful blooms, it will attract bees too.

In fact, there's a very good reason why 
anyone who is serious about helping bees by creating a bee garden will want to include this fabulous flower, and the reason is that scientific data has revealed that Cosmos offers bees a good quantity of nectar and pollen, whilst being relatively long flowering - more about that below.

Cosmos bipinnatus - a feast for the bees, and a feast for human eyes 

Many flowers are valuable for bees because they offer a rich source of quality pollen or nectar.   Seasonal factors, along with issues such as pollinator constancy are also relevant (i.e. the tendency of individual pollinators to visit only certain flower species).

white tailed bumble bee foraging in the center of a deep pink cosmos flower

Nevertheless, some flowers provide both nectar and pollen.

 
Nectar is an energy food for bees, providing important carbohydrate in the form of sugars.

Pollen on the other hand, is vital for its lipid and protein content, and the individual nutrients provided by the various protein amino acids.

How much nectar and pollen do Cosmos provide for bees?

In 2016, Hicks et al published a paper: Quantify the Nectar and Pollen Resources of Urban Flower Meadows1.  A number of flowers were included in the study, and categorized:

  • flowers from a commercial annual flower mix, 
  • a '“Special Pollen and Nectar Wildflower” mix
  • and another group that comprised 'weed' species.

For the purposes of this page, I'm only going to refer to the annual seed mix, since the range comprised plants that are considered 'garden flowers'.

red tailed bumble bee on pink cosmos flower

Below are two tables in which I have compiled all the mean (average) amounts of pollen and nectar for the 12 flowers from the study. 

Whilst 12 is only a small number, nevertheless, the individual data values are given, and anecdotal evidence (observation on my part) would suggest that Cosmos is a valuable and very attractive flower for bees.

Pollen Offering
From the table below, it is interesting to note the following:

  • Cosmos comes fourth on the list for mean volume of pollen per flower per day, behind both poppies (also loved by bees).  
  • However, Cosmos has a longer flowering period than either of the poppies, thus providing an extended foraging period for bees.


Common
name
Scientific
name
Mean volume of
pollen (µl/day)*
Floral unit
longevity
(days)
Common poppyPapaver
rhoeas
5.9582.239
Californian PoppyEschscholzia
californica
2.4073.440
Pot MarigoldCalendula
officinalis
1.8146.706
CosmosCosmos
bipinnatus
1.6928.154
Red FlaxLinum
grandiflorum
0.7071.864
Love-in-a-mistNigella
damascena
0.5495.789
CornflowerCentaurea
cyanus
0.5476.467
Golden TickseedCoreopsis
tinctoria
0.4555.067
Showy Baby’s breathGypsophila
elegans
0.0722.357
Virginia StockMalcolmia
maritima
0.0574.280
TickseedCoreopsis
picta
0.05513.200
Sweet AlyssumLobularia
maritima
0.0062.631

*Mean volume of pollen per flower or capitulum per day (µl/day)

Nectar Offering

For nectar, what we can see is that Cosmos jumps to second place in the rankings (the poppies are ranked lower for nectar, although I still recommend them for their excellent pollen content).

Common
name
Scientific
name
Mean mass of
nectar sugar
per flower
(µg/day)*
Floral unit
longevity
(days)
CornflowerCentaurea cyanus895.8316.467
CosmosCosmos bipinnatus701.0788.154
Pot MarigoldCalendula officinalis469.5156.706
Showy Baby’s breathGypsophila elegans369.2342.357
TickseedCoreopsis picta277.90213.200
Love-in-a-mistNigella damascena240.3015.789
Golden TickseedCoreopsis tinctoria133.4715.067
Red FlaxLinum grandiflorum50.2141.864
Virginia StockMalcolmia maritima46.3494.280
Californian PoppyEschscholzia californica10.3173.440
Sweet AlyssumLobularia maritima4.2132.631
Common poppyPapaver rhoeas0.5702.239

*Mean mass of nectar sugar per single flower or capitulum (µg/day)

From the evidence of both tables combined, not to mention personal observation, I absolutely recommend Cosmos bipinnatus for inclusion in pollinator gardens.

bumble bee foraging on white cosmos flower

Planting Cosmos bipinnatus for bees

I've had pinks and white Cosmos in my garden at various times.  Currently, the space that was previously occupied by Cosmos is at the moment occupied by oxeye daisies, another beneficial flower.

However, having seen the data, it looks like I'm going to have to find a spot and squeeze Cosmos into my garden again.

To grow Cosmos, sow the seeds under glass in mid spring, or you can sow them directly in the soil where they are to flower.  Select a sunny spot.  Deadhead to prolong the flowering season.

These plants are fairly tall (around 1 to 1.5 meters high) and quite bushy with feathery foliage, so ensure they have a good amount of space.

Which bees like Cosmos flowers?

Although the photographs on this page feature bumble bees, Cosmos are also loved by honey bees and a range of solitary bee species.

red tailed bumble bee, side view, or a pink cosmos flower

References

1. Hicks DM, Ouvrard P, Baldock KCR, Baude M, Goddard MA, Kunin WE, et al. (2016) Food for Pollinators: Quantifying the Nectar and Pollen Resources of Urban Flower Meadows. PLoS ONE 11(6): e0158117. 

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