Early Conclusions on Germinating Parajubaea

Angelo Porcelli, Bisceglie (BA), Italy
Chamaerops No.48 - published online 24-05-2004

Parajubaea cocoides enjoying the sunshine in Quito, Ecuador.
Photo: Tobias W. Spanner

The germination of this genus is quite tricky and all who have tried would agree with this. Several ways have been tried, and sometimes very different as they are based on different ideas and speculation on the requirements of this species. Failure has been common. But the method I have been using (after more than one year of trials and observations, reading about habitat conditions, message board interactions, e-mails with many other palm folk sharing opinions and partial results) has given me, at last, a fairly good result.

The first consideration is pretreating the seeds before sowing. It has been confirmed by many that letting the seeds dry for some weeks improves germination significantly. On this I have added cold storage for 4 weeks in a refrigerator at 4°C. This has given me excellent results with palms from temperate climates (Brahea, Butia), which I have sown by the thousands, and recently with a large batch of Trithrinax campestris. The cold storage of seeds is a common technique used by growers of other plant families and, indeed, it is needed for some plants from peculiar habitats (i.e. Sclerocactus and Pediocactus in Cactaceae; many Mesembryanthemaceae-Aizoaceae; some conifers).

After this, the usual soaking in clean water for several days has been followed. I have also added a germination stimulator based on NAA, but, not having a control batch to compare it to, I cannot express an opinion on its efficacy.

At this point, opinion divides. Some growers have used the well-known bag method, which does work well for many species, especially the small-seeded and tropical ones. But with Parajubaea, it has some drawbacks. Firstly, as it is difficult to know when the radicle will appear, it may grow several centimeters in the bag and become difficult to pot up due to its odd shape. This makes it necessary to check the bags frequently, which is not only quite frustrating for the impatient, but also a bit dangerous, as the eventual shoots can be damaged by exploring fingers. But the real problem is that the medium used (peat) allows fungi to grow on fruit that remains trapped in the grooved surfaces of the seeds and this can have devastating results on the seed and emerging radicle.

In the spring of 1999, on a first attempt with a batch of 20 P. cocoides, I sowed the seeds directly into pots, but even this didn’t give me good results. On this batch, I tried to scarify the seeds with physical and chemical methods (filing the shells of 10 and soaking the others in sulfuric acid) but this was a bad idea because, in the end, the other unsprouted seeds became mouldy. Not knowing at which stage each seed was, the watering schedule could not be optimized for both germinated and ungerminated seeds. I ended up with only four plants from this batch and was not satisfied.

I would never consider cracking the shell to sow naked endosperms, as someone suggested with Jubaea or Butia yatay, firstly because it’s almost impossible to crack a P. torallyi shell without damaging the seed, and secondly, the naked endosperms are easily attacked by fungi. This method could only be used in laboratory conditions. On the other hand, Nature has designed the seeds so that they will be spread on the bare ground without killing them and if these seeds have such a hard shell there must be a valid reason. Also, a thick shell doesn’t imply that it is waterproof, and actually the pore from where the radicle will push is almost as thin as paper.

So, I decided to try something different. Perhaps inspired from the seed size, I sowed new batches of P. torallyi v. torallyi, P. torallyi var. microcarpa, P. sunkha and P. cocoides again, using the highly successful method I use with cycad seeds. This consists of using polystyrene boxes with lids and that are 20 cm deep (the kind used to pack cheese, fish etc.), filled with pure perlite to a depth of 10 cm. The seeds are laid on the surface sideways with the point down, buried a third of the way. Then the perlite is gently watered with a fine hose, the lid is put on, and the boxes are put on a shelf 1.90 m above ground in a cold greenhouse covered with clear plastic and 90% shade cloth. This means that temperatures follow the day-night range, with very little difference. The covering doesn’t store much heat at night, so night indoor temperatures are barely a few degrees above outdoor ones. Conversely, daytime maximum temperatures can be significantly higher than outside the greenhouse.

Here, I put a min/max digital thermometer with two sensors, and I have recorded the temperature data inside and outside the boxes. From these data, one thing has become evident. These species like high temperatures to germinate, contrary to the most current advice and despite the montane habitat of the genus, which would suggest lower temperatures. This could also be the case for other genera (e.g., Trachycarpus and perhaps Ceroxylon). I have never recorded any sign of activity when the maximum temperature was under 26°C, and the peak of germination has been in the range 14°C-35°C. This is easily obtained from late spring to early autumn. In the first trial last year, I achieved the following success rates: 11/40 (28%) of P. torallyi var. torallyi, 6/80 (8%) of P. torallyi var. microcarpa,1/30 (3%) of P. sunkha, and 6/10 (60%) of P. cocoides.

When I found no new germination after three weeks, around late October 2001, I stopped misting the seeds. The lids were removed, allowing the perlite and seeds to dry out totally. The boxes were left to over-winter. For a period of over one month, in January 2002, the minimum nighttime temperature was constantly in the range of 1°C — 4°C. In early March, the temperatures rose, fitting into the optimal range. So, the perlite was watered again and the lids put back on the boxes. At the first check, after a week, most of the remaining seeds were germinating: 39/74 of P. torallyi v. microcarpa, and nine more a week later, four P. torallyi v. torallyi and two P. sunkha, and then a wave of 11 P. sunkha at a later check and still continuing. I also found four seeds rotten, easily recognized by the white mould growing on the tip, smelling of rotting coconut. In August 2002, the result was 69 of P. torallyi v. microcarpa, 32 of P. torallyi v. torallyi, and 25 of P. sunka, which means a rate of over 80%. Not bad indeed!

This process seems to imitate habitat conditions rather well—a dry, cool winter and a moist, hot summer—which suits the seeds. Another consideration is that, in the northern hemisphere, we usually get fresh seeds around late winter, while in habitat it is the opposite season. I believe that the poor results I got initially resulted because the seeds should have quite a long period of rest, helpful to induce/remove some chemicals to trigger germination.

The box technique means not having to bother watering the seeds. I just mist them with a hand sprayer when they are dry (the shells turn from dark to light brown). There is no risk of under- or overwatering because, with the lid on, the evaporation is minimal and moisture is held constant. Also, no problem occurs if one overlooks weekly checking for sprouts, as I often do. The radicle grows straight down in the moist yet porous perlite, and no harm will be done pulling up the germinated seed. When the radicle is 4-5 cm long, it’s time to pot it up. I use tall, square pots (10x10x20 cm) filled with a mix of peat/sand/perlite in equal parts. A bit of care is needed for the first watering, as they are sensitive to rot. The first leaf will appear some weeks later.

It’s not uncommon to find seeds with two embryos, especially with the variety torallyi, and this feature has been observed many times in related genera (Butia, Syagrus, Jubaea, Acrocomia, Attalea). Leaving them to grow together could give an unusual effect in years to come, but any attempt to divide them after the germination stage would be fatal for the palm! An easy trick can be used to get two independent plants. Once the radicles are of the right length, I put the seed on the edge of two pots, joined side by side with heavy tape, allowing each root to go in its own pot. A small square pot of 7cm, with the bottom cut off, is placed over the two pots and is inserted with the help of two notches. The remaining space is filled with the mix. This is to prevent the hypocotyls from drying out or being damaged accidentally. After about a year the seed will be spent and the two pots can be separated. And then & two plants for the price of one. Voilá!

All Parajubaea species need to be grown in full sun outdoors from the first leaf stage, at least during the warmest months, as they resent greenhouse conditions (lack of air movement and high level of humidity). Otherwise, they develop weak leaves with yellow stripes, which bend and die slowly. This also indicates the adaptability of this genus to subtropical areas with moist summers. While the ultimate cold hardiness of the several species is still to be well tested, as there aren’t old specimens in cultivation outside habitat, Parajubaea are surely some of the most promising new palms for temperate climates.

Reproduced with permission by the author from the Palm & Cycad Societies of Florida, Inc. Have a look at their award-winning Virtual Palm Encyclopedia for a wealth of information on palms and cycads.

 

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