Germination Technique for Tropical Recalcitrant Seeds
Recalcitrant Seed Germination
There are a few main points we like to keep in mind while harvesting seeds. First is seed maturity, sometimes seeds are not fully mature until the fruit is all but rotting. For most seeds, if it's possible, it is best to let them ripen fully on the tree. We often decide to start picking fruit right at the first signs of rot/mold, or just before that becomes visible. Note that fungus on seed sis generally a bad sign and should be avoided always.
Seeds should also be handled carefully after harvest, ideally with propagation materials on hand for immediate planting. Seeds are best not left out in open air to dry or be exposed to light for long. When cleaning, its best to rinse the seeds and place them immediately into whatever container they are to be germinated in.
Cleaning the seeds
Care should be taken, with some species more than others, that the seed is not damaged, especially near the "eye" of the seeds which is where many species will first crack open and send out their new shoots. Rinsing all the pulp and sugars off the seeds will greatly improve seed success via drastically reducing pathogens.
Generally speaking, and this is far from scientific or accurate around the table, seeds that are hard and shiny tend to have a more robust, impermeable seed coat (e.g. Annona, Miracle fruit) and can often take more abuse via fungal attack or dehydration than those seeds with a softer more porous seed coat (e.g., Durian, Rambutan). Seeds may have hard coats but due to fibers and pulp being very difficult to remove (e.g. Nephelium and Mangifera) the seeds are easy to mold. Some seeds, with thick removable seed coats, such as mango, can be germinated with the seed coat removed. Others, such as Rambutan, where the fibers are very difficult to remove can be germinated in clean moss or with some anti-fungal treatments.
Temperature will depend on species, but this being a tropical and sub-tropical species germination guide, temperatures for most species should be in the 20-30C range. More often than not it is important to keep temperatures constant.
Moisture level of the growth medium for recalcitrant type seeds actually does not need to be that high. This is likely where most fail to successfully grow out their seeds, the medium is too moist. With tropical species like these, water is not often a trigger for seed germination, and thus does not need to be applied like with some other types of seeds. These seeds are primed and ready to germinate as is, usually, and as such the growth medium simply need to be moist enough to maintain seed moisture level and prevent dehydration.
Humidity should be extremely high, this will reduce seed dehydration and allow the tap roots to emerge without any wilt or stress. Note that when using plastics and similar containers, condensation is easy to build up and drip into the growing medium, causing it to become too wet in some cases. Care should be taken to watch the actual moisture content of the growing medium.
Fresh air is a way too underappreciated aspect of botany. High humidity is very hard to maintain in dry areas without sacrificing air exchange, however it is our opinion that it is one of the most important aspects of seed germination. Let eth moss worry about keeping humidity up and let the containers breathe, find new ways of raising humidity without sacrificing important environmental conditions. Grow outdoors wherever the climate will allow for it.
Nutrition is not needed usually until the seedling starts developing a couple sets fo leaves. The seed itself has enough energy in it to allow the seeds to start a small plant, from there it is going to depend a lot on what species are being grown.
Germinating Recalcitrant Seeds
Prepare some clean (without organic debris or mold spots) dry sphagnum moss and soak it entirely in water. You may need to adjust PH of your water depending on your acidity level and plants being grown, but we use tap water without thought. Take the soaked sphagnum moss and squeeze all the water you can in a few second tight fist. Loosen the moss back up, fluff it up, and place it into a bowl with a lid. Use about a 2-4cm layer of moistened moss on the bottom of the bowl or container, depending on the physical size of your seeds.
Place the seeds on top of the bottom layer of sphagnum moss with the "eye" facing down. No need to pat them down, just leave it all very loose. Avoid having the seeds touching each other or the walls/bottom of the container to avoid any wet spots.
Again depending on the size of the seeds, place a 2-4cm layer of more moistened moss on top of the seeds. More layers of seeds can be placed on top, or multiple containers may be used. Note that stacking layers of seeds in one container may make separating them later on, when they are to be transplanted to soil, more difficult.
We often place lids on these containers and fan them 2-3 times daily, but this depends on the species. Sometimes jars without lids are also used with great success, it mostly depends on the species' dehydration risk.
Species we use this method for
Annona sp. - Sugar Apple We use this for many Annona, but it is not usually required.
Artocarpus sp. - Jackfruit, all.
Baccaurea sp. - Tropical Bornean species.
Durio sp. - Durian species, all.
Euphoria longan - Longan
Garcinia sp. - Mangosteen and other Garcinia
Litchi chinensis - Lychee
Mangifera indica - Mango, with and without seed coat
Pachira sp. - Malabar Chestnut
Pouteria sp. - Abiu, Canistel
Rollinia sp. - Biriba
Salacca zalacca - Snakeskin Fruit
Sandoricum koetjape - Santol
Synsepalum dulcificum - Miracle Fruit
Syzygium sp. - Wax Apple, and others
Theobroma sp. - Cacao
Substitutions for Sphagnum Moss
Why is sphagnum moss so ideal? Because of the air spacing it provides. it has an ideal ratio of moisture content and air space inside for many species to grow. So when comparing alternatives, it is often this factor that should be really considered as many soil substitutes tend to e far less "airy" and thus "wetter".
Clay Pellets work wonderful for some species, especially useful in clonal propagation. It is heavier and holds far more water than moss. It will, if rubbed, become mud like and not ideal.
Perlite/Pumice is a very porous medium that is ideal. Despite some thinking perlite and pumice actually hold much water, they don’t. But they distribute water well and have huge amount of air infiltration making it a good candidate. Being rock based, it’s easy to sterilize and doesn’t harbor much in the way of pathogens once cleaned. Treat like an air filled rock, its many air pockets can hold little bubbles of water, millions of them, but it dries out fast as it isn’t truly absorbent.
Vermiculite, unlike pumice, is actually like a sponge and holds massive quantities of water. To avoid rotting and anaerobic conditions, it is advisable that larger grades of vermiculite be used such as 1cm pieces.
Coco can be used, it is best used with hard seed coats such as Synsepalum and Annona as coco in tropical areas is known to have some issues with fungal attack. When moistened and squeezed as in the moss description above, it acts as quite a good seed germinating medium, only mold is the big consideration with coco.