I generally soak old seeds in water for 24 hours with some newer seeds (can be any seeds, tomato, sunflower, cucumber, whatever). Newer seeds release enzymes as they are growing which are released into the water and can help supplement the old seeds. Scarification can help as well.
Nice method and tips though Waveguide.
Or, you take the part of the seed you want out of the seed:
1. scrape off edges with a small kitchen knife
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2. Crack open the shell with your nails or the knife
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3. Leave the green husk on:
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4. Or remove it too. It doesn't matter either way. Plant it in something (soil, coco, rock wool, toilet paper, or aerated water) and watch it grow:
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That excludes the shell being the problem and if it still doesn't grow it's probably dead. No need to wait weeks till enough water penetrated the shell, or the seedling to rot inside cause it can't open the shell fast enough, just open it up.
presoaking seeds in rhyzotonic has worked for me, also soluble seaweed diluted half strength can work, some one said GA3 hasnt worked for them, but i know someone that has tried that method and it has worked for them.