Passiflora Passion flower | Seed Germination

Germination

The Passiflora incarnata seedlings above are a few days old. Having been stored in the fridge for a few months they came up 4 days after sowing. Amazing, as like many Passionflowers, it can be very difficult to grow from seed. Passiflora seed sowing & germination is a complex matter about which little is known for sure. There are many ways to skin this particular cat. For the beginner who does not want to wade through pages of text see below for the basics re seed germination.

Read more about seed germination inhibitors, promoters, pre-soaks & seed storage.

I created these pages many years ago. My colleague Arjen Lommen PhD has kindly provided a 2024 update.

Arjen Lommen (2024) Factors influencing seed germination after reading scientific literature:

Gibberellic acid (GA) is a well-known naturally occurring germination activator in seeds. Abscisic acid (ABA) is a well-known naturally occurring germination inhibitor in seeds. There is an interplay between them. If the ratio of the two goes in favour of GA, then seed germination may start, provided enough water has been taken up (imbibition) and some “imbibition-related bio-reactions (like for instance availability of glucose from polymers)” are underway.

The following are general mechanisms (there may be more) that influence GA and ABA; these, however, are not systematically proven for Passiflora.

1. Other biochemicals in or around the seed may inhibit, but a few might actually activate (for example sometimes some smoke chemicals do).

2. Light : if seeds germinate on top of the earth (for example Aristolochia) then light increases the GA/ABA ratio = activation. If seeds have to be buried (like Passiflora), then direct light decreases the GA/ABA ratio = inhibition.

3. Temperature: too high temp may inactivate seed germination (more ABA); too low temperature slows metabolism and might inhibit too. Low temperature does, however, change the GA/ABA ratio by decreasing ABA and increasing GA; this is in essence preparing for germination for when the temperature goes up in spring.

4. Adding GA during imbibition may tilt the ratio and help germination. The question is how much is actually dissolved in water and also what effect the pH has. Are you using the potassium salt or the less soluble acid … and which is more active?

5. Ethylene gas is emitted by a number of fruits, like bananas and apples etc; this also applies at least to Passiflora edulis fruits. Ethylene helps ripen fruit and has an (in)direct positive effect on seed germination.

6. Drying seeds and many other abiotic or biotic stress forms increases ABA (inhibition).

7. Orthosilicic acid (breakdown product of rock, sand, clay) is known to promote seed germination by increasing GA and decreasing ABA . 

8. Nitrate (but also nitrite and nitrogen oxide gas) may increase GA; these chemicals can have an interplay with light depending on plant species.

From my own experience with Passiflora seeds (more than 100 species germinated), but not water-tight proven:

A. Soak : Most Passiflora seeds have a kind of “membrane layer” on the outside. This often comes off after a clean – soak – clean -soak sequence. For the soak I use a full nutrients solution (this includes nitrate of course) at half strength at pH ca. 5-5.5; nowadays I add orthosilicic acid too at half strength. Cleaning is done by gently rubbing seeds on tissue paper. The membrane layer probably protects (partially) against dehydration, and may also obstruct uptake of water (=inhibition). So, this layer must “dissolve” first. After the second clean I might have dried the seed a bit again, so a second soak is a precaution.

B. Activation by low temp : You can “prime”/activate seeds by 7-10 days at 6 Celcius (even works for tropical passis) after clean-soak-clean-soak (removing out membrane layer).

C. Incubation temperature for germination : This is of great importance and this is determined by the climate the Passiflora species normally lives in. You can use Yero Kuethe’s list to find out the corresponding elevation and approximate location a certain Passiflora was first located and described. Alternatively it is often possible to search on the internet for the particular Passiflora using the phrase “iNaturalist Passiflora XXX”; this will get you to a page where you will find mapped locations where the species was observed in nature. With this info you can check the climate of a village/city close by (phrase “climate village province country”); perhaps correct a bit for elevation. Take a temperature a little higher than the average, but within its normal habitat range.

(UPDATE: For those who need a more computer-view approach the following guide is available on the  Passiflora species Overview page to obtain GPS coordinates of Passiflora species and their corresponding modelled climates. See Passiflora species climate modelling.)

But BEWARE : If seeds are not ripe (fruits not ripe), germination chances are very slim. If fungus has gotten into the seeds, you have no chance of germination. If seeds are old they may have deteriorated or become too dry (for instance not stored properly); chances of germination are then also very minimal.

So … seed germination of Passiflora … is a game in which you try to maximise your chances. If this all too difficult …  just put them outside and wait until by chance they come up because the conditions were right.

Arjen Lommen PhD

Hybrids
Note that new hybrids must be grown from seed of the parental cross, which will be more variable than species seed, possibly with the plant taking more characteristics of the female flower & the male foliage. The best seedlings of hybrids can be named & then cuttings taken to perpetuate them.  If you grow a plant from hybrid seed it may flower within a few months. Species seed may take up to 5-6 years although most will do it in two. Exceptions include P. incarnata, P. foetida & P. tarminiana which may also flower within months. Also some plants grown from seed may never flower. Existing hybrids will only grow true from cuttings.

Technique

Dry seed

• It may be worth filing the seed coat down a bit with a nail file or fine sandpaper, especially along the join, as old seed may have less stored energy in it to help the embryo to burst out of the seed coat. This may mimic seed coat erosion from passing through an animal. I don’t do this on the basis that the seed coat is porous once surface tension is broken.
• Presoak dry seed for 48-72 hours in a small glass of water in a warm location e.g. a sunny window cill. As an alternative you can use orange juice, apple juice, a strong solution of black tea, or fresh Passionfruit juice (my choice) from commercial fruit. As it ferments it may remove germination inhibitors.
• Any viable seed should sink to the bottom of the glass. Throw out any seed that are still floating unless they are rare and precious in which case sow them too.

Fresh seed

• Fresh wet seed will generally give far better germination rates and I just sow it straight from the fruit. That can be either complete with arils, the juicy sacs, or if you wish to remove them first, I find rubbing the seed while wrapped in fine microfibre cloth works well.

Seed potting mix

• I am indebted to Carlos Magdalena for his suggestions. He uses a mix of 4cm deep coir and Perlite with a compost mix underneath to give the seed nutrition once it hatches out. Water well, then place the seed on the top and cover with a further half a cm or 1cm more coir & Perlite and water again.
• I have also tried a mix of reconstituted sphagnum moss (it arrives dried and you just add water) & Perlite and despite, or perhaps because of, looking bit like a mouldy bit of rainforest floor after a while it worked well.

Temperature

• Put the pot in a sealed plastic bag somewhere warm and sunny or in a propagator. 100% humidity is essential.
• If you have a Propagator you have two optimal choices. Either set a constant temperature, I use 80F/27°C, or nighttime 68F/20°C and daytime 86F/30°C. Even higher temperatures may help. Compost heaps can reach 140F/60C!
• Jim Nevers advises as follows, ‘I sow in a mix of 2/3 perlite and 1/3 peat. I set the heat mat at 105F/40C degrees for the first week and 80f/27C for the second week, then continue to alternate if they have not germinated after 2 weeks. I keep them on the misting bench so they have water flow through the mix every day. This way I get some germination even from seeds that are 8 years old.’
• Another simpler method that can work well is to fold some kitchen roll, put it on a plate or in a sealed plastic food container, dampen it and scatter the seed on top then leave either on sunny window sill or in shade. Just make sure that it doesn’t dry out. Once germinated quickly move to a light soil mix with plenty of Perlite in it.

Patience

• Be patient.. wait & hope.. it may take a few days or even many months! Even if nothing comes up you may have done nothing wrong. If all else fails change their conditions, e.g. let them dry out totally, or put in the fridge c/w the potting mix they are in etc. If something does come up gently ease the seedling out of the bag and pot up in a 50/50 mix of compost and Perlite, one per pot as above. Keep it in a plastic bag to start with & gradually roll it down to expose it to the air over a few days. Then place somewhere sunny with good air circulation to begin hardening off. See below. A good principle is to realise that the seed is as desperate as you for it to succeed…getting angry because they fail to germinate or the seedlings topple over will not help!