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The sexing of Hollies - a quick biology lesson. 30 May 2019

The berrying of hollies is a subject that is often broached in discussions with customers and so we thought it might be appropriate to take this as a blog subject, especially as the hollies are flowering right now.  Well they are at Weasdale - everyone else's have probably long finished.

Hollies have imperfect  flowers, that is ones that are either female or male (as opposed to perfect flowers which have male and female parts on the same flower) and most hollies are dioecious, which means that the plants themselves are either male or female.  The next point to understand is that it is the female flower that produces the fruit, (think of the human parallels - it is the female of the species that bear children)  but they need a male flower nearby to fertilise it with the pollen, usually by courtesy of the insect world.

Our native holly Ilex aquifolium, widespread across Europe, is dioecious and so, to the casual observer, appears to berry randomly as the sex of each plant is only obviously female when the berries are carried, whilst the non-berrying ones can be either male or un-fertilised females.  Until you look closer, at the right time of year, providing conditions are right for flowering.  Then you can identify the sex of the plant from its flower.

Here we see a female plant with female flowers.  The female holly flower has a very conspicuous ovary in the centre of the flower that swells after fertilisation to become the "berry".  (The explanation as to why this is in inverted commas will be explained later, so as not to confuse now.)  The ovary is the major part of the flower's female organ the pistil which is topped by the stigma, the part that receives the male pollen and which is connected to the ovary by the style, a short stalk that transports the pollen's sperm to the ovule within the ovary.  The female holly flower carries stamens around the outside of the ovary but, unlike in the male flower, these are infertile and don't carry the yellow pollen-bearing anthers.

In this next image, we can see the male flowers on a male plant.  Compared with the female flower, where the ovary would be, there is a void and the stamens (the thin stalks thrusting out from the flower) carry the conspicuous yellow anthers  that bear the pollen.  Touch these with a fingertip and it will be dusted yellow by the pollen (if still present).

So now we have established that you need male and female fowers within bee- or other insect-flying range (and a bee-keeper will tell you that this can be several miles) to allow fertilisation.  If you want reliable berrying in your garden, it is wise to have male and female plants nearby and so you can aim to plant known-sex plants and one male plant will easily "serve" a dozen female plants nearby.  For a reason not known to your author, all older, European-origin female holly varieties or species have been given male common names:  eg Ilex X alt. 'Golden King' is actually female, whilst Ilex aquifolium 'Golden Queen' is male.  Cross-dressing might be older than you thought!  And then, to complicate matters, the American hollies such as the I. meservae lines, have female names for female plants, so Ilex meservae 'Blue Princess' is as female as she sounds and will need a 'Blue Prince' or similar to fertilise her.

To add a new layer of information, some hollies are monoecious, meaning that they have male and female flowers on the same plant and so will fertilise themselves and so each plant will reliably produce "berries" each year.  Two good examples are Ilex aquifolium 'Pyramidalis' and Ilex aquifolium 'JC van Tol' and these will also act as pollinators for other female species or varieties.

Why was "berry" put in inverted commas you ask?  This is one for Pedants Corner at Christmas, when everyone is admiring the berried holly on the front door.  Technically, the holly fruit is not a berry, but a drupe, like the "berry" on a Yew is also a drupe.  A drupe has a single seed within a hard outer shell, in turn surrounded by a fleshy outer body. Plums, damsons and cherries are other examples of drupes.