Dendrobium bigibbum
There are three main varieties in the Dendrobium bigibbum complex. They are Dendrobium bigibbum var. bigibbum; Dendrobium bigibbum var. compactum; and Dendrobium bigibbum var. superbum.
Distribution
Dendrobium bigibbum var. bigibbum occurs in northern Queensland on some Torres Strait islands, in the east from Cape York to the Archer River and on the western side of Cape York Peninsula from 0 -350m elevation. This is a hot climate with high humidity most of the year with exception of a dry winter and spring with humidity returning in mid-late November. See climate Statistics below for Weipa. Note the main rainfall months and the length of the dry months (very important to keep in mind for culture requirements, see below).
Dendrobium bigibbum var. compactum occurs in north-eastern Queensland on the coastal ranges between Daintree and Cairns (Hartley’s Creek and Mossman River Gorges) at about 350- 750m elevation. This is a slightly milder climate with more regular rainfall but with a distinct long dry- season. See climate statistics for Port Douglass and Mareeba
Dendrobium bigibbum var. superbum occurs in north-eastern Queensland from Cooktown to Font Hills west of Mount Molloy from 5-350m elevation. This is a similar climate to Dendrobium bigibbum var. compactum but with less rainfall.
Climate Statistics
Weipa elevation 18m
John Green highly recommends keeping the pots as small as possible when repotting Dendrobium bigibbum allowing as many roots to grow outside the pot as possible as they need plenty of air around the roots for healthy growth. Use an open mix that will hold some moisture around the roots in the heat of the green-house over the summer months but capable of drying out within 12 hours. It is important to remember their natural environment and to imitate this as best as possible for each variety. The type of potting mix will of course depend on many variables. I use a simple mix of medium grade treated pine-bark with similar sized charcoal and a small addition of coco chip. I mix these items in the following ratio (all by volume not weight):- 45% medium grade treated pine-bark; 45% medium grade charcoal; and 10% coco chip. If your mix is different, it doesn’t matter as these orchids are very tough.
Dendrobium bigibbum var. compactum
Another important point on repotting needs to be emphasised here and that is that because you are only watering these pots for six (6) months a year (see below), the decomposition of the potting mix is slower than normal orchid growing but I still recommend adding lime on the following year after potting and replacing the mix after the second year around mid-November. Stale mix only invites issues with the roots and because they only require small pots the expense is minimal- it’s not worth risking damaging your orchid roots.
In summer we need high temperatures to get good, strong growth. With light watering once weekly commencing in late October till late November-early December. Then water heavily daily till Mid-March then reduce to every 3rd day from mid-March till late April. Then watering ceases until mid-October with light watering commencing again.
Fertilising (mixed at 1⁄4recommended strength) commences around early December, applied daily with watering, ceasing in mid- March. The fertilising regime should alternate between a high Nitrogen (I use Calcium Nitrate) and a high Potassium blend. This will provide lush growth with strong flowering potential in the autumn. It is important to keep in mind that in late October in South- East Queensland minimum temperatures are still only 16-17 degrees C (check the corresponding temperatures in the tables above in October-Weipa: 21.8 degrees C). So don’t be too keen to give them that “Tropical deluge” via the hose just yet.
Remember good growth is dependent on HEALTHY roots and this is what we are aiming for... the plants will do the rest provided the roots are healthy.
Dendrobium bigibbum var. bigibbum
Over winter Dendrobium bigibbum can cope well with cold nights so long as the days are warm, and the roots are DRY. A covered greenhouse provides ideal conditions and also keeps rain off the plants when you don’t need it in winter. Position the plants at the highest place in the greenhouse. Dry conditions are essential with only occasional misting and then only once every 3 weeks or so and only on above average day time temperatures.
They must be completely dry again by sunset!!! This cannot be over-stressed and will stave off too much pseudobulb shrivelling and minimise leaf drop. Keep in mind between May and the end of October the median rainfall is 18mm (Weipa) with each rain day only dropping 0.2-0.5mm. This would barely wet the leaves and barely touch the roots and average daily temperatures are still @ 32 degrees C. Although the other statistics show more rainfall at this time of year the trade winds are strong and very
drying in these locations over the drier months and so create what most of us would consider terrible conditions for orchid growing, yet this is their natural environment. As stated earlier these are really tough plants.
In summary I would like to stress that although I have included a large amount of detail to digest, it really is not hard to have complete success with the Queensland floral emblem in your collection and furthermore many ‘southerners’ will be so envious every Easter when they come to visit to see your beauties blooming profusely and you comment “Oh they are so easy to grow here”. Enjoy your Dendrobium bigibbum collection and you will appreciate how rewarding they really are.
Note: the climate statistics quoted in this article are via the Bureau of Meteorology web site and are copyright for research/discussion purposes only.
Author: David Hunter