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Into Horticulture

Issue 12   March 2013

Plant & Garden News

Honeybees' pollination not as good as wild insects

A study of diverse crop systems around the globe indicates that fruit set was lower in sites with fewer wild insects visiting the crop. Inclusion of honey bees improved pollination, but did not fully compensate for the lack of wild insects, even in crops typically stocked with high densities of bees. Thus measures to increase biodiversity of agricultural systems, such as provision of habitat, could have direct productivity benefits. Source: Wild pollinators increase crop fruit set regardless of honey bees

Is it just salt burn?

In the aftermath of ex-tropical cyclone Oswald, SEQ Catchments are receiving reports of trees in coastal areas badly burnt by wind and salt spray being cut down because they were thought to be dead. Many would have recovered with subsequent rains washing away salt and encouraging new growth. Source: Trees aren't what they seem

American landscape trends

A survey of American residential landscape architecture professionals predict outdoor living areas to be popular in 2013. A preference for sustainability and low-maintenance design are also anticipated. More at the American Society of Landscape Architects website: Survey Identifies Top Outdoor Living Trends for 2013

Mine rehabilitation more than plants

Functional ecosystems include both plants and animals. A study of a rehabilitated sand-mining site on North Stradbroke Island has shown that areas revegetated with the most "success" did not necessarily attract koalas. On the other hand, some areas rated much lower with respect to flora were nevertheless being used by koalas. These findings indicate that fauna should be included when assessing the success of a rehabilitation project. Source: Koalas and mine site restoration

Flowers take charge

New research has revealed that in addition to cues like colour and fragrance, flowers could communicate to pollinating insects by means of electrical signals. Bees develop a weak positive charge as they fly through the air, while plants are usually negatively charged. Measuring the electric fields of petunias via electrodes placed in the stems, researchers have found that bumblebees can detect and distinguish between different fields. Electric fields also helped the bees learn the difference between colours faster. It's possible that flowers could employ this mechanism to inform pollinators on their pollen and nectar status. Source: Floral signs go electric

Sports give trees a sporting chance

A study of Eucalyptus melliodora at the Australian National University shows that mutations that occur during vegetative growth over a tree's long life can lead to some branches being more pest-resistant than others. In this case, the variation was explained by changes in terpene production. These genetic "sports" might be deleterious if carried by the whole plant, but do give a chance of survival under heavy insect attack. Source: Genetic variation controls predation: Benefits of being a mosaic

Tree rings in the tropics

In the past, lack of a distinct hot-cold seasonal cycle has made tree ring data from tropical species difficult to interpret. However, researchers at James Cook University in Queensland have identified some species with good annual rings formed in response to wet-dry cycles. Isotope chemistry has also revealed invisible rings in some species. This and similar studies around the tropical world could reveal new information about the climate in this zone. Source: Tree study may yield climate secrets

e-Clean seeds

A new chemical-free method to kill pathogens on the surface of seeds has been developed in Germany. Low-energy accelerated electrons are used to destroy the DNA of organisms on the seed surface without harming the plant embryo inside. Besides the obvious benefits to humans and environment the avoidance of chemicals presents, this method means that pathogens can't build up resistance. Source: Healthy seeds - treated environmentally friendly

Healthy trees, healthier people

Emerald ash borer, which is capable of killing all 22 species of ash tree in North America, has had a devastating effect in many parts of the continent since 2002. However, it has provided an opportunity for researchers to study the effects of trees our own health. 18 years of demographic, human mortality and forest health data has revealed an increase in deaths from cardiovascular and lower respiratory disease associated with loss of trees in the community. The actual cause of this association is not yet known. Source: Tree and human health may be linked

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The Australian version of autumn is upon us. After many years of drought/bushfires/storms/floods, here's hoping for at least a few months of "just right" weather as we enter one of the best seasons for gardens and gardening.

Local readers might like to know that the 2013 series of Gardening Australia commences on Saturday March 16 on ABC1. Also, 7TWO (Seven Network) have been running episodes of BBC Gardeners World at about 6pm EST weekdays in Queensland (check your local schedules for exact times in your region). Although much of the specific information (e.g. species) won't be applicable in most of Australia, you'll find many basic horticultural principles clearly explained, plus lots of beautiful and inspirational UK gardens to view. How you do think this program compares to our ABC equivalent?

In Issue 9, the value of elite sports as a recipient of public funding that could otherwise be spent on parks and gardens was questioned. Well here's a postscript: The UK's world-famous Eden Project has announced that up to 70 people will be laid off due to a drop in visitor numbers in 2012. This has been partly blamed on the London Olympics [1], even though an increase in tourists had been hoped for [2]. So much for job creation!

For the industry

Do you operate a horticulture-related business in Australia? News and editorials from past newsletters pertaining to marketing or garden industry trends are also archived at Modern Garden Marketing

The Science of Horticulture

Plants and Iron (Part Three)

In this third and final part, we'll look at prevention and remediation of iron deficiency. You'll find the first part of this series in Issue 9 and part 2 in Issue 11.

Avoiding Fe deficiency through cultural practices

As discussed in part 2, various factors can affect the chemical availability and uptake of iron, even when there's considerable amounts of the element in the soil.

In such situations, it may be possible to alleviate or even prevent iron deficiency by altering the conditions in which the plants grow. This is usually preferable to iron fertilisation, for reasons of cost and long-term effectiveness.


If iron deficiency is chronic in a given location, the choice of suitable planting material is the easiest way to avoid problems. If soils are calcareous, this will mean rejecting "acid loving" plants. Species which are native to the area or which come from regions with similar soils will be better adapted. They may have evolved more efficient iron-uptake mechanisms, for example.

Within a plant group, some species or cultivars may be superior to others. Among citrus, both rootstock and scion cultivars vary in their sensitivity to iron shortage. For example, lemons are more likely to develop chlorosis than oranges [1].

With the molecular basis becoming better understood, it's likely that many crops will be engineered for more efficient iron uptake in the future.


Alkalinity makes iron unavailable to plants, so excessive application of lime or dolomite should be avoided. Alkaline irrigation water can also be a problem.

Soil amendments that lower soil pH (e.g. elemental sulfur, ammonium forms of nitrogen, aluminum sulfate, iron sulfate) may be useful in mildly alkaline soils, but impractical at more extreme pH.

Fertiliser interactions

Phosphorus and various micronutrients can interact with iron, so avoid excess fertiliser application. Other sources of metals such as copper fungicides could be potential problems.

Drainage and Aeration, Plant vigour

Poor aeration can impair root function through oxygen deprivation and fostering root rots, affecting subsequent ability of the roots to take up iron. Ensuring adequate drainage or relieving compaction may correct the problem. Iron uptake being an active biological process, anything affecting the number or activity of roots could potentially cause iron deficiency.

Why Fertilise?

As outlined above, iron deficiency can often be prevented or alleviated if other aspects of plant culture are addressed. Nevertheless, some situations may warrant the use of iron fertilisers.

Soils with inherently low iron levels may benefit from supplementation. Containerised plants in soil-free media will, of course, be largely dependent on external iron sources.

In the case of landscape plantings, both the immediate problem of unattractive chlorotic foliage as well as the long-term health and vigour of the plant overall are considerations.

A deep green colour in turf is especially valued. Iron supplementation of turfgrasses for this purpose is common, the nutrient being necessary for chlorophyll synthesis.

Iron Fertilisers

Iron sulfate (ferrous sulfate) is one of the most common forms of iron fertiliser, especially for turf. An additional benefit is its acidifying effect, which may be helpful in some soils (see above). Nevertheless, a lot of the iron applied may be made unavailable if soil conditions are unfavorable.

Chelates hold iron in a plant-available form, avoiding immobilisation by the soil. However, they are relatively expensive and are biodegraded in the soil after a short time. There are many chemical forms. Not all are suitable for high soil pH, becoming unstable.

Note also that iron chelates are sensitive to photo-degradation once dissolved, so growers should aim to protect stock solutions, fertigation tanks, etc from light.

Organic fertilisers may contain some iron, but the quantity will depend on the source of the materials and will require biological action to make them available. Boosting the quantity of natural chelating agents (e.g humates) in the soil is potentially an added benefit of using organic amendments in the long term, although fixation of iron by organic matter may be deleterious in sensitive situations [2].

A variety of other iron compounds are sometimes used in various parts of the world. These include iron oxides, ferrous ammonium sulfate, ferric sulfate, iron oxysulfate, ferric ammonium citrate (iron citrate), iron sucrate and iron humate (a by-product of wastewater treatment). Besides availability, choice of an iron fertiliser will depend on soil reaction and method of application.

Note that some iron fertilisers can stain, so take care with application near walls, paving etc.

Methods of application

Soil Application

Soil application is feasible if there isn't a strong tendency to immobilise iron and/or the quantity of iron required is economically practical.

If using chelates on alkaline soils, be sure the form purchased is suitable for the pH level.

Foliar Application

Sufficient quantities of the micronutrient can be absorbed through foliage to make foliar application worthwhile in some situations.

Unfortunately, the effects are short-lived and largely localised to the treated tissues (recall that iron is not readily remobilised within the plant). Additional applications will be required as new growth appears.

However, it has the benefit that immobilising factors in the soil are by-passed. Also, response is fast. It is especially appropriate for high-value crops at critical phases (e.g tree fruits), important landscapes, or to temporarily alleviate symptoms while more permanent solutions (e.g. improved drainage) are implemented.

Iron sulfate or chelates can be used for foliar feeding. Chelates penetrate better because the organic molecule is not repelled by the leaf's waxy surface as much as a charged ion.

Trunk Injection

Direct injection of iron compounds directly into tree trunks is possible. It might be an option where valuable specimens are involved, but requires special equipment and expertise.

References and Further Reading

[1] Citrus nutrition Agfact H2.3.11, NSW Dept Primary Industries
[2] Phosphorus and Iron Nutrition in Australian Native Plants Australian Native Plants Society (Australia)
For more information about the general aspects of iron nutrition discussed above, see list of links on
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