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Sunday April 25, 2010
Post Title: How to choose fruit trees for your climate  

Choosing the right fruit trees for your orchard or garden has a big impact on how they will grow. How you care for them is very important, but if you start with the wrong tree for your climate, then you could spend a lot of time and money for very little gain. Our Blog this week reveals some key climate related issues to consider when selecting your fruit trees.

You will find that you can plant fruit tree crops in very diverse climates and generally they will grow, but if the climate is not ideal for the fruit tree cultivar you are working with, then it will not be as productive as it should be, be more subject to pests and diseases and ultimately may die prematurely. Healthy, well cared for fruit trees in the right climate can last for a very long time and produce beautiful crops every year.

Let's start with some basics. Fruit tree crops are, for example, apples, oranges and mandarins, whereas fruit tree cultivars of these crops are, for example, Golden delicious apple, or Valencia orange. There are large numbers of cultivars for fruit tree crops., especially the crops that are very commonly grown and consumed. Some cultivars are very old and others are newly developed by fruit tree breeders.

Issue 1 - What are all the fruit tree crops and their cultivars that you could select, as your local nursery or even fruit tree breeder may not have the actual potential range you could choose from? They will normally sell what they have unless you ask them for something else.

Different fruit tree crops have different sensitivity to climate, for example, oranges and mandarins can grow in a diverse range of climates from cool temperate to sub tropical. Whereas bananas like the warmer temperatures of sub tropical and tropical regions and cherries prefer more temperate climates.

Cultivars of fruit tree crops are generally climate sensitive as well, for example, most apple cultivars prefer more temperate climate, but a small number can grow in sub tropical cilimates. Whereas a cultivar like Valencia orange can grow in a diverse range of climates.

Issue 2 - What is the climate profile of all the fruit trees you are interested in and are there cultivars of those fruit trees that suit your climate?

In Australia, there are general climate zones of cool temperate, temperate, warm temperate, cool sub tropical, sub tropical, warm sub tropical, tropical, arid and mediterraneum. Not all fruit tree sellers will use these zones, they may just say, temperate, sub tropical and tropical. You may or may not know your climate zone and such zones are often problemmatic as they cover such broad areas and tend to generalise of lot of different climates. In addition, you may have created a micro-climate on your property where the location of your fruit trees is warmer than the prevailing climate.

Issue 3 What is the local climate profile of your garden or orchard and have you created a micro-climate? Without knowing this, you have no objective way of choosing the right fruit tree

Gardeners familiar with buying fruit trees may be aware of the term high chill or low chill cultivars. Where I live in Brisbane, a sub tropical climate, gardeners may look for a low chill variety of an apple or a peach, whereas a person living in a mountainous part of Tasmania would look for a high chill profile cultivar.

Issue 4 What does chill profile mean, how is it calculated and is it relevant for all fruit trees?

The amount of time that a fruit tree experiences chilling is important for its floweriring process which leads to timely fruit set. This is particularly the case with many varietes of pome fruits, stone fruits, nuts and berries. Yet some fruit trees don't care about chilling, such as the ones growing in tropical climates.

There are two methods of calculating fruit tree chill profile. These are:

(a) Chilling hours This is the total number of hours in a year where the air temperature is below 7c. It is very difficult to get this type climate information from your weather bureau, farmers may use temperature measuring devices to collate this information.

(b) Chilling units This is a measure developed in Australia and is being adopted in a number of countries of the world. It uses the lowest mean temperature in a month of the year, then compares this temperature to a chilling unit scale, for example, in my home town of Brisbane, the lowest mean temperature is in July and its 15 C and this equates to around 190 chill units (low chill). When you reach a lowest mean of 19c, then chill units are 0 and this would be a tropical climate, whereas an average 6c for Orange in NSW would be 1100 chill units (high chill) .

Issue 4 Is it possible to be exact with a chilling unit figure on a fruit tree and who generally calculates them?

Most chill unit figures are quoted as ranges, some crops have very broad ranges, for example a Valencia orange can grow from low to high chill, whereas with Apples, very few will grow in low chill below 250, but nearly all cultivars will grow fine from 300-1000 chill units. The ranges are most often provided by the fruit tree breeders, that's if they use chill units. They may instead completely ignore chill factors and use zones and say, for example, this cultivar is suitable for sub tropical only.

Issue 5 If you use chill units as a measure, how do you know what your chill unit profile is so that it can be matched against the fruit tree cultivar.

For most gardeners, this is a stab in the dark as they would not know how to calculate their chill units or they may use chill hours instead of units and the hours, as mentioned previously are nearly impossible to calculate unless you have a device to track temperatures 24/7 for every day of the year and do this over a number of years.

Issue 6 Where does this leave you as a gardener attempting to make objective decisions?

It leaves you in a very tricky situation. For persistent gardeners it will take a lot of time to get the bottom of these questions. This is impacted by the complexity of the impacts, the information is located in many places and often has its own particular bias. You could join a local fruit tree growing club. This should be a storehouse of local knowledge, depending on how well it is run.

How does our web site solve these issues for fruit tree gardeners?

(a) We store the climate profile of the 40 most common fruit tree crops with over 700 cultivars of these crops covering all climate zones of Australia. These profiles have been converted into chill unit ranges for all fruit tree cultivars (cultivars for tropical climates will have very low or zero chill factors). We are progressively adding more fruit tree crops and cultivars. We do not sell fruit trees and will list cultivars from any breeder, thus giving you a very broad picture of the potential crops to choose from.

(b) We store local climate information in our web site and when you choose your nearest weather station for your orchard, the web site creates a chill unit profile for your garden and can be further tuned for your micro climates.

(c) When you search a fruit tree for your climate, all the cultivars will be listed which match your chill profile and that of the fruit tree. Where our web site logic finds that your chill unit profile sits within 20% plus or minus the mid point of the cultivar chill unit range, the cultivar is marked as Ideal to plant and outside this mid point range as OK.

(d) You can then easily access the fruit tree breeders against each cultivar (Australia only at present) and contact them directly or ask your local nursery to buy from them on your behalf.

The search is very simple to use and will also show you whether the cultivar is an early, mid or late cropping variety, so if you were planting oranges and wanted to extend your cropping season, it would be best to have three trees which were early, mid and late varities. The search also shows dwarf varieties which is very userful if you do not have much space.

You can see how it works by clicking here. Based on our research, this fruit tree search is the first of its kind in the world.

Peter Kearney, www.cityfoodgrowers.com.au

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Peter Kearney, Cityfood Growers

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