Thursday 17 January 2019

Warm days hasten work of pollinators

Warm weather speeds up pollination - and means flowers bloom and die more quickly, writes Gerry Daly

Flowers are producing less nectar during the heatwave
Flowers are producing less nectar during the heatwave

People have observed that spring flowers, such as rhododendron and tulip, did not last very long in recent weeks, when some really warm days arrived. The theory expressed is that the flowers shrivelled in the heat due to lack of moisture. But there were plenty of days when significant amounts of rainfall occurred. Nor has the soil become suddenly dry such a short time after being more-or-less at full field capacity - the maximum quantity of water that any particular soil can hold.

While there were some quite windy days and these would have dried out and knocked down some flowers that had done their job, windy days would not have been the primary reason: that flowers do not last once pollination has been achieved.

The sole reason that plants have evolved to make flowers is to attract agents of pollination: insects, tiny humming birds and even bats. The eyesight of these animals varies and many South American plants have deep red, green or yellow-orange flowers, while insects are attracted by white, yellow, mauve and magenta flowers, and reflected ultra-violet light, unseen by the human eye, plays a big role.

Warm days speed up the work of pollinators and they get round to more flowers more quickly. As soon as the plant gets the chemical confirmation that the flower pollination or fertilisation has occurred, the petals and other disposable flower parts are shed, sap closed off. Only the seed-forming ovary remains. Wind soon knocks the now-redundant flower parts to earth where they rot down, and their nutrients are recycled. Plants invest considerable resources in making flowers with petals, scent and sweet nectar.

Some plants, for instance, grasses and most conifers, are wind-pollinated, having fewer flowers, not colourful, but these require the production of vast quantities of light pollen. It is the wind-blown pollen that causes hay-fever, not the flowers with petals that are most often wrongly implicated. Insect-pollinated flowers, such as most garden flowers, have heavy sticky pollen that attaches to the agent of pollination and is carried to other open flowers.

Bees and other flying and crawling insects tend to visit the flowers of the same species while they are open and the nectar is in full flow, making it easier to collect. It also encourages the insects to visit the same kind of plant; there's no point in delivering pollen to the wrong tree. The nectar flow lasts for as little as a few days and another species takes over. The synergy between bees and plants works to the advantage of both, but it is also true that the incidental, pollen-carrying activities of insects allow plants to shorten their flowering season. The popular cries of 'good for pollinators' and 'increase pollinator numbers' also inevitably means shorter flowering periods.

Luckily, many garden plants are hybrids, some infertile and incapable of successful seed-making. Japanese spring cherries are an example, losing no energy in the production of fruit and seeds, diverting instead into a massive show of blossoms. It is ironic that the annual symbol of life should be infertile and incapable itself of passing on life. Lots of garden flowers are double-flowers with extra, usually narrow petals, formed from stamens as mutant forms, but with more colour impact.

Why go to all this trouble to swap pollen within species? Living things have DNA that controls the expression of living tissue. If the DNA is re-bonded only with itself, flaws appear and may be expressed as disease conditions. Crossing different populations of the same species allows the damaged or inadequate DNA to be masked by healthy DNA. It is so essential for continued success that plants and animals invest great resources into it, the whole process driven by evolution.


Hip HIP, HOORAY!: Three cheers for Irish gardener Billy Alexander who, fresh from his Silver Gilt Medal win at the RHS Chelsea Flower Show for his 'Wilde Atlantic Garden' of rare and exotic ferns grown and matured at Kells Bay House and Gardens, has just scooped a win at Bloom for Best in Show for Nursery & Floral Pavilion Awards for his wonderful tree-ferns.

Visitor from Overseas: Among many fine plants that grow in gardens is the spectacular, orange-red Chilean flame tree, or embothrium. Being native to Chile, and evergreen, it is not fully hardy and does not survive inland, needing the warming effect of the sea coast, especially in the south, where the best specimens are grown. When happily established, it can form a bushy tree to over 5m.

Cultrual Gardens: Those disappointed that there will be no Ballymaloe Litfest this year can console themselves with a visit to their 5th Outdoor Sculpture Midsummer show instead. Richie Scott and Hazel Allen will host one-hour long guided tours of the sculptures on Tuesday and Thursday nights at 6pm throughout summer, from June 21. For more, see or

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