To make a simple isolation cage, you need some cheap nylon fly-screen 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 of netting1m by 5m will give a cage large enough to cover 3 or 4 plants.   

Cut a square piece of screen 1 metre by 1metre to make the top of the cage, and then fold the remaining strip of fly-screen 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.

Aubergine

Broad beans will cross pollinate with other broad bean varieties nearby, so if you want to keep your variety pure you will need to isolate your plants.  But if you are unconcerned about some cross pollination you can save seeds from plants in the middle of your bean patch.  Always save seeds from strong and healthy plants. Let your broad bean seed mature and dry on the bush.  The pods will turn dark brown, dry and wrinkled.  Then pick and shell them.  Check that they are really dry, and if they are not, dry them further in the sun in a good flow of air.  Broad bean seeds should keep for two years but if you want fresh seed grow plants for seed every year.

French and Runner Beans

It is important to grow some bean plants specifically for seed, rather than simply collecting the left-over pods at the end of the season.  The plantas should be good strong specimens and any that are less healthy looking or not true to type for the variety should not be used for seed productions.

French beans are self-pollinating but if you are just saving seed for your own use, grow your seed crop of french beans at least 2 meters away from any other variety (4 Metres if possible) and you are unlikely to have a significant problem with cross pollination. Runner bean flowers need to be 'tripped' by wind or insects before the beans set, and are much more likely to cross with other varieties grown nearby than french beans.

To collect the seeds, allow the pods to mature fully on the plan until they start to yellow and dry out.  Then spread out somewhere in the sun with a good airflow until the pods are fully dry and brittle.  Once they are dry, shell out the beans and dry further out of the pods.  Store in an airtight container.  If they are well dried and stored in a cool dark place, the beans will last around 2 years. 

If you have problems with weevils eating your seeds, put the sealed container in the freezer for a week immediately after drying the beans, this will kill any insect eggs before they hatch.  When you take them out, let the container come up to room temperature before opening it, otherwise the beans will absorb moisture from the air.

Brassica

Sprouting broccoli, cabbages, cauliflowers, kales and brussels sprouts are all members of the same family and will all cross with each other. They won't cross with turnips, swedes, oriental brassicas or mustard greens. So as you only seed save from one member of the family in any give year, you can grow as many other brassicas as you like without problems so long as you don't let them flower.

Carrots

Carrots are biennial, flowering in their second year of growth.  Foliage will die back but will then re-sprout and start to flower in the spring. 

Carrots grow into big plants producing successive branches with large flat umbels of flowers. 

To harvest your carrot seed, keep an eye on the umbels of flowers, and cut them off with secateurs as they start to turn brown and dry.  If you have plenty of plants, just save seed from the first and second umbels of flowers to appear on each plant, as these will give the biggest and best seed.  Dry the seed heads further inside, and then rub them between your hands or in a sieve to separate them.  You will notice that the seeds have a 'beard' which is removed in commercial seed to make them easier to pack. You can sieve the seeds further to remove more of the chaff but there is no need to get the seed completely clean, just sow more thickly with the chaff.  Carrot seed can be stored in a cool and dark place for up to four years.

Lettuce

Lettuce flowers are self pollinating and very rarely cross.  If you plan to save seed from more than one variety, separate them by around 4 metres or plant a tall crop between the lettuce rows.  

Select two or three good lettuces from your row, and mark them for seed.   It is very important not to save seed from any plants that bolt early, as you want to select for lettuces that stand well. Heading lettuces may need a little help for the flowering stalk to emerge; slitting the heads partially open with a knife works well.

Once the lettuces have flowered, the seeds will ripen gradually, starting in about a fortnight.   Harvest seed daily to get the maximum yield,  shaking into a bag. Or wait until a reasonable number of seeds are ready and then cut the whole plant.  Put it head first into a bucket, shaking and rubbing to remove the seeds  If you leave the whole cut plant upside down in the bucket somewhere dry, slightly immature seeds will continue to ripen over the next few days.  

Most of what you have collected in the bucket will be white 'feathers' and chaff.  To sort the seed, shake it gently in a kitchen sieve.  Some seeds will fall through the sieve, with the rest collecting in the bottom.  The feathers and chaff will rise to the top, and you can pick them off.  There's no need to get the seed completely clean; a little chaff stored and planted along with the seeds won't cause any harm.

If the seed feels a little damp, dry it further on a plate before labelling and storing.  Lettuce seed should keep for around 3 years, provided it is kept cool and dry. 

Melons and Cucumbers

All varieties of melon will cross but cucumbers won't cross with melons, but will cross with any other cucumbers or gherkins nearby.  It is possible, although fiddly, to hand pollinate both melons and cucumber flowers.  Grow plants under a fleece tunnel to exclude insects, and then hand pollinate the flowers on those plants with a paintbrush.  Make sure that you exchange pollen between different plants to keep the diversity of your variety.  

To harvest melon seed, pick the melons when they are ripe and ready for eating and keep indoors for a further day or two for the seed to mature further.  Then open the fruit, scoop the seed out, and wash in a sieve under running water.  Spread out on a plate to dry thoroughly. 

Cucumbers need to be ripened well beyond the edible stage.  They will become much fatter with green varieties will turn a dark yellow brownish colour and white varieties a paler yellow.  Keep for a week or so after picking to let the seeds mature fully.    Then cut open, scoop out the seeds and surrounding pulp into a jar, add a little water and stir well.  Leave  the jar on a sunny windowsill for 2-3 days for the seeds to ferment.  On the third day, fill the jar fully with water, and stir well again.  The good seeds should sink to the bottom of the jar, leaving pulp, debris and empty seeds floating on top.  Gently pour off the water and debris, refill the jar and repeat.  After a couple of rinses you should be left with good seeds at the bottom of a jar in clean water.  Drain off the water and spread out on a plate to dry well.

Both melon and cucumber seeds will last for 6  years if dried well and stored somewhere cool. 

Onions and Leeks

Onions and leeks are biennials and will not usually cross pollinate, so just grow as a normal crop but keep aside some plants and allow them to produce flower stalks in their second year. Once flowered, watch carefully as once mature the seed pods can easily shatter. As soon as you can see the black seeds within the drying flowers you should cut off the head and allow to continue drying in a paper bag. The seeds will fall to the bottom of the bag but gentle shaking will speed the process. Seeds left in the flowers can be removed by rubbing the flowers between your hands. Allow the seeds to dry for a period before storing and ripe onion and leek seeds should keep in a cool and dark place for up to two years.

Parsnip

Parsnips will cross-pollinate between varieties easily so save seed from one variety each year. Parsnips produce a tall flower stalk in their second year, so leave a number of roots in the ground. Pull up any roots which start to flower before the others or you will be saving seed prone to bolt. Parsnips produce little yellow flowers on their stalks which are followed by huge numbers of papery seeds. Harvest and allow to dry for a few days before storing in a cool and dark place. Parsnip seeds are best used within a year of harvesting.

Peas

Peas are almost entirely self pollinating, only very occasionally crossing with other plants.  Set aside a section of row that is entirely for seed production.  Tp avoid physical mixing up of the seeds, separate different varieties of pea with another crop.  Check the row from time to time as the peas grow and pull up any plants that  are weak or not true to type.

Let the peas mature until the pods are brown and the seeds start to rattle.  Once the pods start to wither, dry  them further in the sun.  Once the pods are really dry, shell the peas out, label with the variety, date and store in a cool and dark place.  Pea seeds will keep for two years.

Pumpkins, Courgettes, Marrows and Squashes

Beware that pumpkins, squashes, marrows and courgettes will all cross readily with each other.  The best way to save pure seed on a home scale is to hand pollinate one or more fruits. This is very easy and will avoid disappointments with lcrosses.   The explanation given here is for pumpkins, but applies equally to squashes, courgettes and marrows.

Pumpkin plants have two different types of flower, male and female.  The female flowers are the ones that will grow into pumpkins. They can be identified by the small immature fruit which should be obvious beneath the flower.  Male flowers just have a straight stem. You need to transfer pollen from a male flower into a female flower, making sure that no pollen gets introduced from plants of a different variety.

One evening, when the plants are just beginning to produce flowers, find some male and female flowers that are going to open the next day. Buds that are just ready to open are much fatter than the others, and they have turned from green to yellow.

You need to stop these flowers opening, so that insects can't get into them.   The easiest way to do this is to gently slip a thin rubber band over the end of the petals, to hold them shut.

The next morning go back to the plants.  Pick a male flower, take off its rubber band, and tear off the petals. Gently take the rubber band off of one of your female flowers. Using the male flower like a brush, rub the pollen on to each section of the stigma in the centre of the female flower.

Then carefully rubber band the female flower shut again so that no insects can get in with more, 'foreign', pollen.  Tie a piece of wool loosely around the stem of the female flower, so that at harvest time, you know which pumpkins you have hand pollinated.  

Now leave the pumpkins to develop and ripen. After you have harvested them, keep them in a cool dry place for another month or so to ripen further indoors.

Then cut the pumpkin in half, and scoop out the seeds, leaving the rest of the fruit for cooking as normal.   Wash the seed in a colander, rubbing it between your hands to get rid of the fibres, and then shake off as much water as possible.

Spread the seed out on a plate to dry.  It needs to dry as quickly as possible, but without getting too hot, for example on a sunny windowsill. Seeds stored in a cool and dark place will keep for 6 years. 

Radish

Radish varieties will cross-pollinate so try to save seed from only one variety per season. Radishes are easy to save seed from and will produce seed the same year that you sow them. Keep aside one or two healthy plants and and allow them to produce seed stalks. The stalks can grow surprisingly large and bushy so allow sufficient room, about a 45cm, for the stalks to fully develop. The seed pods become pale brown and the stalks should be cut down when both the pods and stalks are dry and hard. The seeds can be removed either by cracking open individual pods or grating the pods through a sieve and then winnowing away the chaff to leave the seeds in a stiff breeze. Allow to dry for a few days before storing in a cool and dark place and your seed you last up to five years.

Sweetcorn

Leave a couple of healthy and plump looking corns on a healthy and disease free plant for seeds. Cover your chosen cobs with paper bags and tie around the corn to prevent insects entering the bag. Remove the corns after allowing them to dry thoroughly, peel back the husks and hang somewhere, insect and rodent free, to dry completely. The seeds can be extracted by twisting the cob between your hands and picking out any remaining plant debris. Allow the seeds to dry for a few days before storing in a cool and dry place for up to two years.

Sweet Peppers and Chillies

Sweet peppers and chillies are both members of the same 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 toisolate your plants by around 50 metres from any other peppers or chillies growing nearby.

If you want to grow several varieties 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.   

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 and store in a cool and dark place. Capsicum and chilli seeds will keep for five years.

Tomatoes

Most modern varieties of tomato are self pollinating and will not cross.

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 but a

Oriental brassica and turnip are sub varieties of the same family as turnip. This means that although they will cross with each other, or with turnips in flower, they won't cross with broccoli or cauliflowers.  Although you can only grow one of these vegetables for seed in any year, you can of course grow any of the others for kitchen use, so long as you don't allow them to flower at the same time as your seed plants. 

These plants are naturally biennials, producing their flowers and seeds in their second year of growth. Plants will usually flower in there second autumn but will need to be shaded and kept moist to survive the summer.  Select at least 6 of the healthiest and most typical plants to reserve for seed, eating the rest.  In the second autumn, the plants will flower, and then form seedpods. 

The seedpods are green at first, but then gradually dry out and turn a pale tan colour.   Once most of the pods are dry and brittle, cut the entire stalks of the plant, and lay out on a sheet somewhere undercover with a good airflow to finish drying off.   Then rub and crush the pods with your hands to release the seeds, and separate the seeds from the chaff with a coarse sieve. Seeds can be stored for 3 years in a cool and dark place.