I usually sow sweet pea seeds in the autumn. October seems to be the best month. I do not nick, sand or any other way mutilate the seed. I find that they germinate well without damaging the outer shell of the seed. I think that if the seed do not germinate, it is more to do with the viability of the seed rather than the thickness of the seeds outer coat. The pots are left in the greenhouse overwinter. It is a cold greenhouse with no heating at all.
I sow the seed in individual 3 inch pots and label them carefully. I use New Horizons peat free compost to plant them in.
When the seedlings have grown their second or third leaf, I pinch out the growing tip to encourage side shoots. The strongest side shoot is left while all the others are removed. Some growers leave two side shoots. From what I have seen of other sweet pea growers, this is quite an important thing to do if you are growing exhibition standard cordon sweet peas.
In the past I have taken out a trench and put in well rotted compost or manure, however now the soil is very fertile on the allotment I don't necessarily do this any more.
I use canes to grow the sweet peas up and make a line of canes in a similar way to runner bean rows.
I plant sweetpeas out fairly early - end of March or the beginning of April. When I planted the sweet pea seedlings I included both inoculated charcoal and mychorrhizal fungi in the planting hole.
I grow them as cordons i.e I take off all side shoots and tendrils and tie them to a cane. It is really the only way to get large flowers. The other secret, and please don't tell anyone else, is to feed the sweetpeas with comfrey liquid. They love it.
I am experimenting with using charcoal inoculated with comfrey tea and that had even better results for me. We will be experimenting again in the spring.
The leaf is the food factory for the sweet pea. If you take off the leaves it will begin to starve to death. The tendrils grow out of the leaf and these can be taken off - not the leaves. If you are cordon growing them then you will need to take off the tendrils and the side shoots. They are then grown up strings or canes to support them. I am going to put some pictures of my sweet peas on the blog later. You will also have to tie them onto the canes with wire because you have taken off their natural climbing apparatus. If you do this carefully and feed them with comfrey liquid you will get large exhibition flowers.
If you just want flowers for the house then let them grow naturally and provide them with netting to climb up themselves. They will not grow as large as cordon sweet peas either in height or size of flowers.