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The Natural Life Cycle for Tulips

Tulips originate from the mountains of Iran and Turkey where the climate varies from hot and dry in the summer to extremely cold and severe in the winter.    Because the climate is generally too severe for perennial plants to survive, various mechanisms including bulbs make possible survival from year to year.

Bulbs are essentially energy stores which allow the plant to survive from the end of one growing season until the beginning of the next.   At the end of the growing season the energy in the bulb is stored as starch.   This is a good way of storing carbohydrate energy but is not available to the plant until broken down into sugars.   It is under the influence of cold temperatures that hormones are formed which convert the starch to sugars.

(The transformation of starch to sugar is readily seen in potatoes which, although not bulbs, are also energy stores.   In potatoes we wish the carbohydrate store to remain as starch because if the starch is converted to sugar then fried potato chips will turn black.)

The normal annual cycle for tulip bulbs is as follows:

          SEASON                        BULB DEVELOPMENT

1. Autumn rain                  Develop roots
2. Cold winter                   Convert starch to sugar
3. Early spring                   Plant grows using sugar reserve
4. Mid-spring                     Flowering
5. Late spring                    Carbohydrate stored in the new bulbs
6. Early summer                Bulb dormancy

Warm Winters

In Australia there is generally insufficient length or coldness in the winter to fully effect the transformation of starch to sugar.    As a result the flowers will generally emerge less evenly and flower much shorter than would be the result had the winter been more severe.

This situation is exacerbated in areas with very warm winters (frost-free) where there may be insufficient cold to promote normal growth in the bulb.

It is the existence of such climates in Australia and the desire to grow tulips that has led to the idea of refrigerating tulips before planting.

Refrigeration

It should be noted that refrigerating before planting is not part of the natural process.   Tulips normally receive their cold period after they have developed roots.   As such it is not a natural process and there are three primary effects of this process:

a)       the bulbs will flower about 1 month earlier than uncooled bulbs

b)       the stem length at bud opening will be taller

c)       the bulb yield will be reduced

It should be noted that while the stem length at bud opening is less without cooling the final stem length at petal fall may be similar to the cooled bulbs since stem growth continues until petal fall.

Weeks of refrigeration are cumulative and the more weeks given the greater the above effects will show themselves.    Any period up to about 8 weeks is possible for gardeners but 6 weeks is better.    Also, the temperature of cooling also affects the results. Cooling at about 9 degrees C will give less manifestation of the above symptoms than cooling at lower temperatures around 5 degrees.     Cooling in the refrigerator at about + 2 degrees is probably preferable to 5 degree cooling but less desirable than 9 degree cooling for bulb production. Even better is no cooling at all, if that is possible.     Long pre-cooling can produce lovely flowers but in combination with the leaf area loss at picking can devastate bulb production.

The effect of refrigeration on bulb yield can be substantial.   Long refrigeration, say 10 weeks, can result in reducing weight increase from 100% un-refrigerated to near 0% increase.   For this reason it is recommended that planting stock (say, 1ess than 10 cm) should never be refrigerated as the main purpose of planting this is to achieve weight of bulbs rather than flowers.   The effect of refrigeration on reduction of bulb yield is cumulative as noted in the previous paragraph.

When to Refrigerate

The above discussion would indicate that pre-cooling bulbs is not particularly desirable unless it is necessary for the climate in which they are grown.

For home gardeners, as a general rule of thumb, if you have winter frosts then you do not need to refrigerate at all.    All of Tasmania and much of the inland areas of Australia, especially up the Great Dividing Range as far as southern Queensland do not require any refrigeration at all.   Refrigeration may be used to make the tulips flower earlier and this is cumulative up to about 8 weeks of chilling.

For much of coastal mainland Australia some chilling is required for tulip production although the amount required depends on the variety of tulips and the location.   A group of tulips known as single lates (sometimes endowed with generic commercial names by suppliers) generally require less chilling and some members of this group will even flower in Sydney with no chilling at all.  However, about 4 weeks is recommended for these in coastal southern Australia and 6-8 weeks north of Sydney.

At the other end of the spectrum are the Darwin hybrids, especially the old fashioned Apeldoorn which requires considerable cold and is not recommended for northern Australia at all and 6-8 weeks chilling in coastal and warm areas in southern Australia.

Refrigerating for Flower Production

Where flower production is the aim the ability to manipulate tulips through the use of temperature pre-treatment is very useful and is the basis of programmed forcing of tulips.   The ability to produce long stems on early tulips is generally very desirable in the market from a price point of view.     For flower production, cooling periods up to 12 weeks are possible.

Cold Requirement and Tulip Varieties

As an approximate rule, tulips can be divided according to cold requirement as follows:

• high cold requirement                     darwin hybrids
• moderate cold requirement              triumph, early tulips
• low cold requirement                      single late tulips

The use of single late tulips in warmer climates definitely enhances the chances of successful production.

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