Aubergine flowers are mainly self pollinated, but can be crossed by insects. So if you are planning to save seed, you should only grow one variety. Aim for 6 to 8 plants each year to maintain a variety long term. For 100% isolation you need 50 feet between your seed plants and any other aubergines. If you are growing them in a greenhouse/polytunnel you should be able to get away with a somewhat smaller distance.
To get ripe seeds let the fruits mature well past eating stage. Purple/black cvs turn a muddy purple-brown colour, green/white cvs turn yellowish. Mark 1 or 2 early good fruits on each plant to leave for seed, and then pick and eat later fruits.
To remove the seed, cut into quarters lengthwise, avoiding the core, and pull apart. The hard brown seeds should be obvious. Put the quarters into a bowl of tepid water, and rub the seeds out with your fingers. You may need to pull them apart to get all of the seeds. Add more water, stir thoroughly, & wait a few minutes. Good seeds will sink to the bottom, leaving debris and poor quality seeds on the surface. Pour the debris off gently through a sieve, then refill with water and repeat a couple more times.
Eventually you will be left with good seeds in plain water. Empty into a clean sieve, shake to remove as much water as possible, and then tip on to a plate and spread out well. Put to dry somewhere warm but not hot, and mix occasionally to make sure that they dry evenly and don’t stick together. Aubergine seeds will keep up to 7 years if dried thoroughly & stored in a cool dark place.
Sweet peppers and chillies
Sweet peppers and chillies are both members of the same species, Capsicum annuum (some less common chillies come from other capsicum species).
Pepper flowers are self pollinating, and will set fruit without any insect activity. However, they will also cross readily, and sweet peppers will happily cross with chillies. You need to isolate your plants by around 150 feet (50 metres) from any other peppers or chillies growing nearby. Even if you are only growing one variety be careful about other varieties growing in adjacent gardens or allotments.
If you want to grow several varieties, or if your near neighbours are also growing peppers, you could consider making an isolation cage to cover 3 or 4 plants. This is easy to do, and costs very little, especially if you can get hold of some old net curtain material. You can put a cage up over plants grown in pots, growbags or directly in the ground.
To save the seed, take peppers on your isolated plants which have ripened fully to their final colour (usually yellow or red). Cut the peppers open carefully, and rub the seeds gently off of the ‘core’ onto a plate. Wear rubber gloves to deseed chillies, as the chilli oil sticks to your fingers and is very hard to wash off. Dry the seeds in a warm but not hot place until they snap rather than bending
Making an isolation cage
To make a simple isolation cage ideal for peppers or aubergines, you need some cheap nylon flyscreen 5 times as long as it is wide, four canes or thin stakes, and some string and garden wire. Alternatively, you can use old net curtains, or other netting small enough to exclude insects. A piece of screen 1m by 5m will give a cage large enough to cover 3 or 4 plants.
Cut a square piece of screen 1m x 1m to make the top of the cage, and then fold the remaining strip of flyscreen round and sew its ends together. The resulting band will be the sides of the cage. Then sew the top to the sides, making a cube of flyscreen with the bottom missing.
To put up the cage over your plants, hammer the four canes into the ground in a square a little smaller than the cage top, so that they stick up a little less than the height of the cage. Twist a short piece of wire tightly round the top of each cane, and then run string in a square around the tops of the canes, supported by the wires to stop it slipping. Run a second piece of string around the stakes lower down to stop the sides of the cage blowing in against the plants. Then slip the cage over your plants, and weigh it down with earth or rocks.
Most modern varieties of tomato are self pollinating, and will not cross. The anthers on tomato flowers (which make the pollen) are fused together to make a tight cone that insects cannot enter. Usually the stigma (the receptive surface for receiving pollen) is very short, and so is located deep inside this cone of anthers. No insects can get to it and the only pollen that can fertilise it comes from the surrounding cone of anthers.
In a few varieties however, the stigma is much longer, sticking out beyond the cone of anthers. In this case, insects can get to it, and there is the chance of cross-pollination. Varieties with longer stigmas include potato leaved tomatoes and currant tomatoes. To avoid crossing only grow one variety with exposed stigmas. The double flowers which are sometimes formed first by many beefsteak tomatoes also often have exposed stigmas, but later single flowers will be normal.
To collect the seed, allow your tomatoes to ripen fully. Then collect a few of each variety that you want to save seed from. Slice them in half across the middle of the fruit, and squeeze the seeds and juice into a jar. You then need to ferment this mixture for a few days – this removes the jelly-like coating on each seed, and also kills off many diseases that can be carried on the seeds. To do this put the jar of seeds and juice in a reasonably warm place for 3 days, stirring the mixture twice a day. It should develop a coating of mould, and start to smell really nasty!
After 3 days, add plenty of water to the jar, and stir well. The good seeds should sink to the bottom of the jar. Gently pour off the top layer of mould and any seeds that float. Then empty the good seeds into a sieve and wash them thoroughly under running water. Shake off as much water as possible, and tip the sieve out onto a china or glass plate (the seeds tend to stick to anything else). Dry somewhere warm but not too hot, and out of direct sunlight. Once they are completely dry, rub them off the plate and store in a cool dry place, where they should keep well for at least 4 years.