Tuesday, February 11, 2014

Better Pollination, Bigger Crops

Knowledge of how pollination works can make the difference between getting a crop or no crop at all. It can also help you achieve a bigger crop than you otherwise would have.  So let's have a look at some of the theory behind pollination and then at how this can assist you in your food garden.

Plants that produce a fruit, for instance tomatoes, peas and apples, require pollination in order to develop that fruit.  Eighty percent of flowers on our planet have the structure shown below.

Pollination is the process by which pollen is transferred from a stamen to a pistil.  Successful pollination starts the production of fruit at the ovule, the base of the pistil.

How is pollen transferred from a stamen to a pistil?  The four major pollination methods are:
  1. Transfer by insects (bees, wasps, ants, beetles, butterflies, moths, flies), birds or bats: 80% of all plants use this method of pollen transfer ; flowers are often brightly coloured and scented and provide nectar meals to attract the pollinators.
  2. Transfer by wind: used by sweetcorn, grasses, conifers, maple trees and other deciduous trees ; flowers are often dully coloured, unscented and have small or no petals since pollinators do not need to land on them.  They often have long stamens and pistils.
  3. Transfer by humans: for instance when creating hybrids ; hybrid plants now form a significant percentage of the total number of plants on the planet.
  4. Transfer by water: used by aquatic plants which release their pollen into water.
Sweetcorn is pollinated by wind.  Planting sweetcorn not in a row, but in blocks of at least four plants increases the rate of wind pollination, making it more likely that all corn kernels on each cob will develop.

Now let's look at an average garden where there are a number of flowering plant varieties and multiple plants of each variety.  Insects and wind and birds will bring all sorts of pollen to many pistils.  Will all these pollen transfers lead to successful pollination and therefore fruit?

Scenario 1: a pistil receives pollen from a plant of a completely different plant family
Pollination will fail.  In the rare exceptions where it is successful a new plant variety is born if the offspring grows successfully and produces its own viable seed.

Scenario 2: a pistil receives pollen from a plant of the same plant family
If the pollen comes from a plant of the same variety or if the variety is different, but a member of the same plant family, successful pollination will take place.
This is called cross-pollination.

Scenario 3: a pistil receives pollen from the same flower or another flower on the same plant
Eighty percent of flowers on our planet have all the parts shown in the diagram above.  This means that the nearest pollen to a pistil is the pollen in the same flower.  Many plant and trees varieties will only develop fruit if cross-pollination takes place.  However, some plant varieties can successfully pollinate themselves, either by using pollen from the same flower or pollen from another flower on the same plant.  This is called self-pollination.
Self-pollination is often achieved by wind or vibrations insects create when they visit a flower and which result in pollen within a flower 'wafting' to its pistil.

Most self-pollinators also develop fruit if cross-pollination takes place.
Most cross-pollinators do not develop fruit if self-pollination takes place.

When you decide to have a new fruit-bearing plant variety in your garden it is important to know whether it is self-pollinating or cross-pollinating.  Why?  Because if you get just one capsicum plant (a self-pollinator) it will bear fruit.  However, if you get just one Apple tree, and you don't have another one and there is no other Apple tree close by in a neighbouring garden, you will get no fruit whatsoever because Apples are cross-pollinators.

A good example of a self-pollinator is pumpkin. Pumpkins are among the 20% of plants that do not have flowers with both male and female parts as in the diagram above.  Some pumpkin flowers only have stamen (male flowers), whereas others only have pistils (female flowers).  Pollen from a male flower can successfully pollinate a female flower on the same plant.  You can leave it up to insects to do this, or be much more certain that you will get pumpkins by doing it yourself.  The process is described on this blog at http://foodgardengroup.blogspot.com.au/search/label/pumpkins
A female pumpkin flower has been pollinated and a pumpkin begins to form
Using seeds from this year's pumpkins to grow next year's crop is easy and saves you having to buy seed, but observe one rule: don't plant another variety of pumpkin near the pumpkin variety you want to plant again next year, because the result might well be that next year's pumpkins become a mix of the two varieties.

Now let's have a look at a situation I had in my garden a month or so ago.  In October I put eight capsicum plants in a little unheated hothouse.
Nice and warm when the weather was cool 
We had an unusually cool spring in Tasmania and the capsicums definitely liked the increased temperatures.  They grew well, then began to flower, but none of the flowers resulted in fruit.  Was that because the temperatures in the hothouse were not quite warm enough?  I don't think so.

Capsicums are self-pollinating, but that does not mean they do it all by themselves and you don't need to know how they do it.  The term means that if you have one Capsicum plant you will get capsicums because the plant will use its own pollen.  However, this will only happen if it can use one or more of the four pollination methods!  I should have opened the roof of the little hothouse during sunny periods or taken it off completely when the first flowers opened so insects and the wind could do their work.  I have now taken the roof off permanently and plenty of capsicums are forming.

Tomatoes are also self-pollinators.  Commercial growers of tomatoes in hothouses either place insects inside the hothouse or use special vibrators to gently shake each flower in order to achieve successful pollination.

Those of us who have a hothouse and who want to grow tomatoes or capsicums when there are no insects around (perhaps because they can't get into the hothouse ; yes, open a window) may want to shake or flick flowers or use an electric toothbrush as explained in YouTube video http://lifestylevideos.com/how-to-hand-pollinate-tomatoes/ .
Do this when the pollen is not likely to be wet from dew or irrigation.

Of course you can also use these methods to make every flower result in a fruit and therefore maximise your crop if your fruiting plants are outside.

Apples and Pears are cross-pollinators, but the story is a bit more complicated than buying two of the same variety Apple or Pear tree: for best pollination results an Apple or Pear tree needs to have near it an Apple or Pear tree that is a different variety, and of course the two trees need to blossom at the same time.  When matching Apple and Pear varieties experts consult information based on successful field-trials. Consult your trusted nursery or talk to someone with expertise on fruit trees during our food garden visits.
Woodbridge Fruit Trees has a pollination guide on its web site and notes on pollination requirements for each variety (see http://www.woodbridgefruittrees.com.au/woodbridgefruittrees/content/8-pollination-guide )

Apricot trees are self-pollinators, but often benefit from having another Apricot variety nearby.

Here are some examples of self-pollinating fruit-bearing plants:
Tomatoes, capsicums, peppers, tamarillo, pumpkins, peas, beans, lettuce, eggplant, sunflowers, wheat, barley, oats, rice, nearly all common varieties of apricot, peach, nectarine and sour cherry

Here are some examples of cross-pollinating fruit-bearing plants:
Almonds, Pecan nuts, grapes, most Apple, Plum, Sweet Cherry and Pear varieties, Raspberries, Blackberries, Strawberries.  Notable exceptions are Satsuma plum and Bartlett pear.

It would be great to extend these lists further as more information comes to hand.  It would be great to receive info from readers on this subject.

The info contained in this blog post is also of interest to those who are into collecting seeds from vegetables.  If you are, a good article to read is Pollination of vegetable crops at http://www.caes.uga.edu/publications/pubDetail.cfm?pk_id=6828

Thank you, Marg M, Margaret W and Max K for reading my draft of this blog post and Margaret W for providing additional info.

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