Difficult Seeds To Germinate: 3 Types (Explained)

Germinate seeds

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It can be frustrating to plant your seeds and they just don’t seem to germinate after days and even weeks of waiting. 

Seeds that are difficult to germinate either have a hard seed coat (e.g. avocado, peach), have a light requirement (celery, lettuce), or have a temperature requirement (e.g. parsley, carrots, lettuce). 

Be sure to check out these three types of seeds and their examples to help you understand the requirements some seeds need to germinate successfully and more quickly.

Let’s get started.

Seeds with hard coats

1. Avocado

Avocados have difficult seeds because it has a hard outer seed coat that is impervious to water and oxygen, to prevent the seed from germinating prematurely when conditions are not met.

Germinating Seeds In Rockwool Cubes

Depending on the growing conditions, avocado seeds can take 2 months on average to germinate.

Germination tip:  To speed up an avocado seed to germinate, the best way according to the California Avocado Society (1988) is to peel off the brown seed coat and also to slice half an inch (1.3 cm) off the top or apex of the seed.  Then,

Physically removing or cracking open the hard outer coat of seeds is called scarification, a technique to allow water and oxygen to enter the seed.

After the pre-germination treatment, put three toothpicks into the seed to immerse only half of the seed in a glass of water.  This way is better than germinating the seed in potting mix.

2. Peach

Peaches are stone fruit with seeds that are true to type.  The seed is inside a very hard and thick casing which is impervious to the water and oxygen in the outside environment.  Because of this, it can take between 1 to 3 months to germinate.

Germination tips: Cold stratification can help germinate the hard seeds of peaches more quickly.  First, dry the hard seeds in the sun for 2 weeks before putting them in the freezer for 3 months and then planting them in soil.

Another way to make the hard seeds germinate faster is to remove the hard casing (scarification) or crack them open using a hammer or a similar tool.

3. Sweet peas

Depending on your soil temperature, sweet peas can take 7 to 30 days to germinate.  Sweet peas can take a while to germinate because they have a hard exterior.

Another reason why sweet peas take a long time to germinate is that they have “expired” or lost their viability after being stored for more than 2 years.

Germination tip:  Their hard seed coat can be softened by leaving them in a damp environment overnight, e.g. by misting them with water but not completely submerged in water, or by simply planting them directly in soil that is not waterlogged.

Seeds sensitive to heat

Temperature is considered one of the key factors in germination success and rate.

Seeds with a temperature requirement are generally perennials or cold-season plants which are adapted to germinating in cool temperatures.  High temperatures would inhibit such seeds from germinating, a phenomenon called “thermo-inhibition”.

4. Parsley

Parsley seeds are sensitive to heat and germinate best in the fall and winter.  It is difficult to germinate parsley seeds under warm temperatures in summer.

According to a study, there is a high germination rate of parsley seeds at temperatures between 10 °C and 25 °C (between 50 °F and 77 °F), with the highest success rate at 20 °C (Silva et al., 2017).  Above 86 °F (30 °C), the germination rate reduces significantly and is close to non-existent above 95 °F (35 °C) (see Table 1). 

TemperatureGermination rate
50 °F (10 °C)76.5%
68 °F (20 °C)89.25%
77 °F (25 °C)69.25%
86 °F (30 °C)26.5%
95 °F (35 °C)0.25%
Table 1: Germination rate of parsley seeds at different temperatures (Source: Silva et al., 2017)

It is believed that thermo-inhibition applies to the plant species of the parsley family (Apiaceae) which also includes carrots and coriander. 

5. Carrots

Carrots seeds are also sensitive to heat and their optimal germination temperature is 68 °F (20 °C).  Temperatures above 86 °F (30 °C) inhibit the germination of carrot seeds

A study has found that carrot seeds not only suffer from thermo-inhibition, but also “thermo-dormancy”.  When they are incubated or stored at a high temperature of 95 °F (35 °C) for just 1 day prior to germination, only 35% of the seeds germinated even though the germination temperature was then returned to the optimal of 68 °F (20 °C) (Nascimiento, 2007).

In contrast, carrot seeds that were incubated at a low temperature of 59 °F (15 °C) for 3 days prior to germination all germinated at the optimal 68 °F (20 °C) and even at the adversely high temperature of 95 °F (35 °C).

Germination tip: Carrot seeds should be kept in a cool place away from the sun and should be planted in late fall and early spring when the temperature is between 59 °F and 68 °F (between 15 °C and 20 °C). For best results, plant carrot seeds ¼ inch (6 mm) below the surface of the soil.

6. Strawberry

As a cold-season perennial, strawberry seeds are also sensitive to heat.  

Depending on the variety, they grow as perennials in hardiness zones 5 to 8, but can be grown as annuals in warmer zones 9 to 10 because of a lack of winter with very low temperatures.

Germinating strawberry seeds can take between 1 to 6 weeks, depending on the germination temperature.  Because they are adapted to growing in cool temperatures, they do not germinate unless they have gone through a period of low temperature before spring comes. 

Germination tip: To mimic their natural cycle, the technique of cold stratification can be used to make them germinate more quickly.  Put the seeds in a ziplock bag and in the fridge for a month before planting them in damp soil.

You can also put them in the freezer but the seeds first need to be dried for a week in the sun. This is because seeds with moisture will suffer from freezing injury in subzero temperatures.

7. Coneflower

Coneflowers, also known as Echinacea, are difficult to germinate because they have a temperature requirement to break seed dormancy.

They are cold-season perennial flowers with a large cone-shaped center. Coneflowers grow in hardiness zones 3 to 9 and are native to Western Minnesota, Montana, and south to Texas and New Mexico. They grow native in a wide range of wide-open areas like meadows, and on the edge of wooded areas.

Germination tip:  Coneflower seeds need to be cold stratified before germination to mimic their natural cycle. They should be kept in the fridge for 2 weeks to a month. Once taken out they take about 10 to 15 days to germinate.

Seeds sensitive to darkness

8. Lettuce

Lettuce seeds are sensitive to heat as well as darkness.  In other words, they require light to germinate. 

They are cold-season vegetables and grow in all seasons, except summer, in hardiness zones 4 to 10. They can take 3 weeks to germinate.

Seeds that are sensitive to darkness contain germination inhibitors to prevent them from germinating under a lack of light and high temperatures. Light is necessary for lettuce seeds to germinate because it induces the production of nitric oxide (NO) and phospholipase D (PLD) which are essential regulators for seed germination.  NO in turn promotes the production of the plant hormone gibberellin (GA).

When plant hormones cytokinins and gibberellins are used to remove the germination inhibitor, lettuce seeds can germinate in darkness.

Germination tip:  For home gardeners, giving celery and lettuce seeds adequate light can help them germinate faster.   This can be done when they are only lightly buried in soil, or without being covered. Also, high temperature and too much moisture in the soil would inhibit their germination.

9. Celery

Celery seeds also require light to activate certain plant hormones to germinate.  When they germinate under light, they can tolerate a higher temperature. 

The temperature tolerance for germination depends on the cultivar.  According to a study of the seeds of a celery cultivar Lathom Blanching, they can germinate at a higher temperature of 26 C when put under light (Biddington & Thomas, 2006). 

In contrast, the seeds could not tolerate high temperatures and can only germinate at 16 C when the germination took place in darkness. 

Conclusion

For seeds with a hard coat, they can be easily removed or opened up using a tool.  For seeds that are sensitive to heat, care should be taken with the temperature of seed storage.  Seeds are best stored in a cool place away from direct sunlight to prevent overheating.       

Happy gardening!

Related

Does Freezing Affect Seeds? (Here’s How)

5 Ways To Break Seed Dormancy: Which is the Best?

How Long Do Seeds Last? (And How to Store Them)

Top 12 Fruits That Grow True To Seed

References

An, Z. F. & Zhou, C. J. (2017). Light induces lettuce seed germination through promoting nitric oxide production and phospholipase D-derived phosphatidic acid formation. South African Journal of Botany, Vol. 108, pp.416-422

Biddington, N.L. & Thomas, T. H. (2006). Thermodormancy in Celery Seeds and its Removal by Cytokinins and Gibberellins. Physiologia Plantarum, 42(4): 401 – 405

California Avocado Society (1988).  The Effect of Pretreatments on Avocado Seed Germination. Yearbook 72: 215-221

Nascimento, W. M. (2007). Preventing thermo-inhibition in carrot by seed priming. Seed Science and Technology, 35(2):504-507

Palevitch, D., Thomas, T. H., & Austin, R. B. (1971). Dormancy-release of Celery Seed by a Growth Retardant, N-dimethylaminosuccinamic Acid (Alar). Planta, 100(4), 370–372.

Silva, T. A., Baldini, L. F. G., Ferreira, G., Nakagawa, J.,

Silva, A. A. (2017). Thermoinhibition in parsley seeds. Bioscience Journal.

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