Flowers and pollen.
A honeybee, covered in Dandelion pollen.
Plants in nature employ a fascinating array of strategies to ensure pollination of their flowers. When they first evolved wind was the only agent that could use to move pollen from one plant to it’s neighbour, and ancient lineages of plants such as cycads, ferns and conifers still make do with drift pollination to this day.
Much, much later on came the flowering plants. The myriad forms, colours and patterns of flowers that now exist, and in which we gardeners take such delight, all developed for one single purpose: to ensure pollination.
Flowers do this, of course, by using their none too subtle signals to attract an equally large galaxy of insects, birds and mammals to carry out their work for them.
Clouds of Pine pollen released on the wind.
This has proven to be an incredibly effective strategy and, as a result, flowering plants are able to reproduce themselves despite only producing tiny quantities of pollen.
Your average conifer, by contrast, consigns truly industrial quantities of pollen to the air each spring in the hope that a minuscule proportion of it will land in exactly the right place at exactly the right time, to ensure successful pollination.
It’s fair to say that most wild plants do successfully manage the trick of pollination in most years and are thus able to set seed and attempt to reproduce themselves.
Pollination in the garden.
An Aquilegia - a typical hummingbird pollinated flower: red and with elongated nectaries.
In the garden however, things do not always go so smoothly. Almost all of the plants that we grow as ornamentals and for food are well outside of their wild environments, and often totally isolated from their natural pollinating partners.
Not too many of us have hummingbirds visiting our gardens for instance, yet it’s they that are the wild pollination partners for a number of our red-flowered garden favourites such as Aquilegia.
Which is where hand pollination comes in.
Hand pollination.
Hand pollination is simply the pollination of a plant by “mechanical” means – i.e. by you the gardener. There are three good reasons why you might want to hand pollinate flowers in the garden.
Tomato flowers.
Firstly, many fruit and vegetable plants (tomatoes being a prime example) often need help in pollination to ensure a good crop of fruit develops.
This is particularly so when the plants are grown under glass where any potential pollinators may not be able to reach them, and where the plants may well be flowering artificially early, before most pollinators are on the wing.
Secondly, you may wish to ensure a crop of seed is produced that you can successfully harvest for sowing or distribution. In the absence of natural wild pollinators some plants will only set seed if given this additional help.
A mature stigma with a few pollen grains already attached.
In addition, in order to limit the number of genetically identical copies of themselves, many plants have developed various strategies to prevent self-pollination.
These plants are referred to as being self-sterile (heterothallic, if you want to get really technical) and they will need artificial assistance from a second donor plant in order to fruit well or to set seed at all.
Thirdly, you may, like me, wish to have a go at playing god and try to make your own hybrids. This can be extremely rewarding, and it’s certainly great fun watching new babies develop and seeing how they differ from, and hopefully improve upon their parents.
Techniques and strategies.
Anatomy of a Tulip. Dark brown pollen on the anthers of the male stamens which surround the white female stigma in the centre.
The simplest form of mechanical help that you can give a plant is to simply give it a gentle shake. Some fruit and veg plants that are self fertile (but that suffer from a lack of insect pollinators) can benefit greatly from this as the loose pollen is easily dislodged from the stamens and transferred via gravity to the stigma. A number of glasshouse crops are pollinated in just this way and electric “shakers” are used commercially on a vast scale.
Of course this isn’t much help of you’re looking to create a hybrid or to be certain of pollination of a particular, individual flower, so this is where true hand pollination comes in.
Although the anatomy, structure and size of flowers differs hugely, the basic reproductive elements are present and correct in all of them.
Simply put, dust-like pollen grains are produced on the anthers of the male stamens and need to be received by the female stigma.
Once on the stigma the pollen grains will germinate and grow towards the ovary where they can fertilise the waiting seeds.
Hand pollination tasks.
Lily stamens and stigma.
The stages of the hand pollination are:
1) To correctly identify the stamens and the stigma in the intended flowers. Sometimes it’s blindingly obvious what’s what, but other floral anatomies, especially in small flowers, can be a bit more cryptic. The internet can usually help.
2) To figure out when the stamens are ripe and shedding pollen, and when the stigma is receptive. This is crucial.
Stamens are of no use unless they are shedding fluffy, light pollen. They only do this for a few days, so timing is important.
The stigma becomes moist and sticky when receptive, and most also change shape and curl outwards, all ploys to try to trap pollen.
Normally the stigma ripens first and then the stamens mature some time later (although some flowers operate the other way around) so you will need to keep an eye on several different flowers and use each at the right stage.
Pollen grains on a tulip stamen.
3) Transfer clean pollen from stamen to stigma. Traditionally this has always been done with a very fine artists paintbrush…which I personally think is a terrible idea.
Firstly much of the pollen is lost either in the bristles or in the act of moving from flower to flower. More importantly, once used the brush is then tainted with the pollen of that flower and needs to be sterilised with alcohol to kill the pollen residue before being used on the next variety of flower.
Unless you are only pollinating one variety of flower this is totally impractical and some flowers produce such tiny quantities of pollen that almost none would be transferred on a brush.
Hand pollinating an Iris using tweezers.
Another popular technique uses cotton buds. These are pretty good at collecting pollen and can be discarded after each plant, so there’s no danger of pollen contamination.
In my experience though much of the pollen is again lost in the fibres of the cotton and some of the flowers that I hybridize - Epimediums for example – are far too small for a cotton bud to be of much use.
By far the simplest and best idea is to use a pair of long handled, narrow pointed tweezers to pluck the ripe stigma and us it to “paint” the stigma with pollen. If the flower parts are large enough, you can simply use your fingers instead of tweezers. This way you get the maximum possible amount of pollen transferred and there is absolutely no possibility of contamination. Easy!
Painting the stigma of a courgette flower using a plucked stamen.
4) If you’re attempting to hybridize you must prevent anything else from pollinating the flower afterwards. There are two aspects to this.
Firstly you’ll need to prevent the flower from pollinating itself. The surest way to do this is to simply remove all of the stamens before they ripen and shed their pollen. This process is called emasculation, and is easily done with those same long tweezers.
Secondly you need to prevent any insects from visiting the flower.
If you are pollinating a small-ish plant then you might want to have it in a pot which will allow it to be moved indoors for a few days until whilst the stigma is still receptive.
Alternatively you can hood the flower, to prevent anything gaining access to it. Paper bags secured with cotton or cotton are ideal, and allow good movement of air and humidity, which is crucial if the pollen is to germinate successfully. The third option is to remove all of the petals from the flower so there’s simply nothing left to signal to an insect.
A honeybee, covered in Dandelion pollen.
Clouds of Pine pollen released on the wind.
An Aquilegia - a typical hummingbird pollinated flower: red and with elongated nectaries.
Tomato flowers.
A mature stigma with a few pollen grains already attached.
Anatomy of a Tulip. Dark brown pollen on the anthers of the male stamens which surround the white female stigma in the centre.
The simplest form of mechanical help that you can give a plant is to simply give it a gentle shake. Some fruit and veg plants that are self fertile (but that suffer from a lack of insect pollinators) can benefit greatly from this as the loose pollen is easily dislodged from the stamens and transferred via gravity to the stigma. A number of glasshouse crops are pollinated in just this way and electric “shakers” are used commercially on a vast scale.
Of course this isn’t much help of you’re looking to create a hybrid or to be certain of pollination of a particular, individual flower, so this is where true hand pollination comes in.
Lily stamens and stigma.
Pollen grains on a tulip stamen.
Hand pollinating an Iris using tweezers.
Painting the stigma of a courgette flower using a plucked stamen.
