Archive for May, 2011

If one thing annoys me more than any other it’s the much overused phrase Australian native plants. Often used in the context that they are universally wonderful in all respects. And the inference is that they are all the same.

You mean to tell me that a eucalyptus that comes from a high rainfall area in Tasmania should be treated the same as one that comes from the goldfields area in Western Australia? Or that a banksia from Perth’s gutless sandplain, the same as one from the Stirling Ranges, or the swamps of Sydney?

And of course none of them need fertiliser, they are all water efficient and totally resistant to pests and diseases because they are native and perfectly adapted to here! And neither will we mention pruning. 🙂

What a load of hooey!

Lets look at fertiliser requirements first. There are a whole host of Australian native plants that ARE quite phosphorus intolerant. In fact some are quite intolerant of fertiliser per se. But they aren’t all Proteaceae such as banksias and grevilleas. And some of them will quite happily grow in a (relatively) high phosphorus environment if they are started out that way because they don’t develop the specialised proteoid roots that make them take up phosphorus so efficiently. But not all of them, and we don’t really know which are which – the amount of research that’s been done on native plant nutrition is pathetic. That’s not to criticise what has been done its more a comment on how much more there is to do and how poorly these plants are understood. Even the mechanism by which the phosphorus is dealt with in the plant, differs between species – its actually not always inhibition of iron – in at least one species of verticordia it has to do with disrupting sugar metabolism. And amongst the verticordias there’s a huge range of tolerance to fertiliser. V. plumosa will grow quite happily in quite a saline environment with high fertiliser whereas others like V. eriocephala (the cauliflower one)or V. cooloomia, won’t. Most Myrtaceae such as Eucalyptus respond well to normal garden fertiliser.

What about irrigation? Well because the dryland (eg northern sandplain) plants are adapted to summer drought, and indeed often go into a sort of summer dormancy, they are also structured to make the most of water when it is around and they will suck it up like nobody’s business if given half a chance! Unlike many traditional plants that shut their stomata down to regulate water flow, many of our native plants don’t. So they will take what you give them and grow accordingly even if they aren’t usually meant to grow at that time. Many believe this actually shortens their lifespan but it also helps keep them looking presentable in summer!

Then you have plants like boronia – heterophylla (a red one) and megastigma (the highly perfumed one) which come from down the south west of WA where its acidic, peaty and damp. And you wonder why they don’t thrive in your garden in Perth. They need moisture – plenty of it and a cool root run.

And as for pests and diseases. Unfortunately because ‘native’ plants were born here they have a whole suite of pests and diseases that have also grown up around them and adapted to them. So they are often prone to more pests and diseases than introduced species. Dieback of course is introduced and that is also why its so devastating – because our plants HAVEN’T evolved to live with it.

So next time you buy a native plant (or indeed ANY plant really because all plants were once native to somewhere) for your garden think about the fact that it may have come from the subtropics of Queensland or the Kimberleys, the mountains of Victoria or Tasmania or even the northern sandplains of WA. It may normally grow in impoverished sand, in crevices on granite rocks, in iron rich clay or peat swamps. If it grows normally as an understorey plant it may hate wind, conversely it may need plenty of sunlight so it doesn’t grow leggy.

Bottom line- don’t expect to treat all native plants the same. And don’t think that they all appreciate compost and fertiliser in the planting hole – it may be absolutely the WRONG thing to do. They may not appreciate either the fertiliser OR the decreased level of aeration around their roots. The same goes for a thick layer of mulch.

And can you plant stuff when its 40ºC? Yes. Provided you can keep the water up to it. And provided it has had some acclimatisation first – that is it didn’t come from under shadecloth or in a greenhouse. So if its sun hardy and you can afford to water it (depending on the size of its root ball) up to 3-4 times a day you should be fine. Remember a little and often in sand because it stores so little moisture. The kindest thing you can do to any plant you plant out is get its roots established quickly out into the surrounding soil. Commercial growers plant all year round and they may even water hourly for the first 7-10 days purely to get that good initial root establishment which is so critical to nutrient and water uptake and therefore growth (and for them yield=$$$).


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Well its cooler and there are showers around. Can I stop watering? And should I race round with soil wetting agents to fix the problem?

There are always two things to consider when it comes to watering. Well, fundamentally two things. The plant and the environment. Lets consider the environment – more specifically evaporation. Back in January and February the evaporation per day was about twice what it is at the moment. Say 8.5 versus 4.0 mm per day. On the hottest days in January it got up over 11 mm. So we are still losing about 4 L water per square metre of ground. So lets consider our plants. They may or may not be actively growing at the moment. If they are they will be using water. Remember we’ve had a couple of fairly heavy showers too so some plants may already be trying to grow and its important that we support them. If they are growing they will be sucking up water from the soil and drying it out and that moisture will have to be replaced or the new roots and shoots that are forming will die.

There are a few other traps at this time of year. If you’ve been watering with drip there may be a salt build-up around the edge of the wetted zone in the soil. The first showers of rain that come down may push some of that salt back into the root zone and so it is possible for plant roots to be damaged by that salt and you may see signs of that on the plant itself such as scorching round the edge of the leaves.

Irrigation is all about the water being where the plant can get to it – that is where its roots are. So if you’ve been watering through a dripper in one place, don’t suddenly move it to another because most of the roots will be back where the dripper WAS and your plant may die from lack of ACCESSIBLE water.

A really interesting paper was published in an irrigation journal last month. The authors looked at a range of wetting agents (five in all – granular and liquid) to see how well they worked on our sandy soils in Perth. Now bear in mind this is a refereed paper using good scientific methodology so the results are worth listening to. They found that only a few days after applying the wetting agent, soil wettability had declined back to OR EVEN WORSE than its previous level. Their conclusion was that many products DO NOT enhance long term wettability, in fact they even seem to make the original problem of non-wetting worse!

Food for thought? Back to clay amendments I think.

BTW the reference if you want to hunt it up is: Gross A, R Mohamed, M Anda and G Ho, 2011. Effectiveness of wetting agents for irrigating sandy soils. Water, April 2011, 154-157.

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What are your choices?

Well I suppose there are slow/controlled release products like Osmocote, Nutricote etc etc. Then there’s things like NPK Blue, Nitrophoska etc which we call compound fertilisers because they contain a mixture of nitrogen, phosphorus and potassium plus a range of trace elements. There are liquid fertilisers – the ones you add a teaspoon or 10 mL to a bucket of water. Many of them are compound, that is they have a range of nutrients in them and often trace elements as well.

Look down any shelves in Bunnings or your local supermarket and you will also find rose, citrus, camellia etc fertiliser. Then there’s blood and bone and superphosphate – shortly to be outlawed for use on Perth sands due to their high phosphorus content. OK for clay soils, not for the Swan Coastal Plain. And lets not forget things like Seasol/seaweed/fish and all those other organic type products.

Where to start? Well The first place to start is probably at soil pH. Once you’ve established you’re about in the normal range ie 6.5- 7 you at least know that what you put on will be available to the plant.

The next thing you’re probably asking is do I need to put on different fertiliser for different plants? Well for the most part, no. Perth sands do tend to be deficient in magnesium so its important to see that well represented in any product you buy and citrus tend to need quite a bit so watch out for that. There are vege crops that need manganese and boron in above-average amounts so that is also good to bear in mind but boron especially can easily become toxic and some plants don’t like it so be careful. If some is good, more is not necessarily better!

Watch out for the ratio of nitrogen (N) to other elements like phosphorus (P) and potassium (K). Most plants want 5-10 times as much N as P. That’s why many organic fertilisers are not good on Perth sands – they have far too much P in relation to N. You end up putting more and more on to get good growth and in doing so leach P into the groundwater because what your plants are really after is N! Even potatoes which are said to need heaps of P still need plenty of N and in fact need about 5:1 N:P.

N:K is important. Nitrogen gives good growth and lush foliage but it can be at the expense of the plants immunity to pests and diseases – lots of yummy soft growth! Too much K and stems can become brittle. Everyone says you need K for flowering but that’s not really true. Plants need all elements at all stages of growth. Too much N though and they may go vegetative and NOT flower.

Many of the “organic fertilisers”, especially the liquid ones don’t contain enough actual nutrition to make plants grow unless you use lots. And I mean lots. And that means $$$.

Because plant nutrition is such a minefield you have to have your wits about you. Especially with liquid fertilisers because there is such a variation in the content of the product, how it is made up and how much you apply. And bear in mind that applied to any potting mix, or to sand, the NEXT time you water, all that is left is apt to go straight out the bottom and leave your plants hungry till the next time you feed them! Controlled release fertilisers are a much better option in most cases. Even NPK Blue lasts a week or more.

Things worth bearing in mind. A commercial crop of celery needs about 50-60 g nitrogen per square metre. Lettuce in summer 30 g per square metre nitrogen. Citrus – 25 g. Now if your fertiliser is 12% N, for example, that means divide what you need by 0.12 to get the actual grams of fertiliser needed. The answer may suprise you!

What are YOU giving your plants and how much are you flushing out of the root zone before the plant can grab it? A study of a whole range of different fertilisers on turf, in Perth showed that it really didn’t matter what you fertilised with, it was how well you irrigated that determined the end result. Fertilising and irrigation CANNOT be separated. I have seen growers lose 90% of what they put on by bad irrigation practices. I would say many home gardeners do the same (if they fertilise). And many don’t. Remember folks, in Perth we are hydroponic. We are growing in a medium that offers little more than support. But also remember that most organic fertilisers don’t retain nutrients in sand either, they wash out the bottom just as well as most other fertilisers unless there’s a reasonable amount of clay around.

Bottom line for the most part – use a fertiliser with a full range of nutrients in reasonable proportions and apply little and often.

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