Beetroot, chard & leaf beet
Beetroot, leaf beet/perpetual spinach, swiss chard & sugar beet are all members of the same family & will cross readily. They are biennial, and flower in their second year. Chard/leaf beet for seed are overwintered in situ, and will be fine in most of the UK. Select a minimum of six to eight plants to leave for seed which best fit your needs (depending on your preference for stem versus leaf, smooth or wrinkled leaves etc). Beetroot can also be overwintered in situ, or can be harvested in autumn, the best plants selected & stored then replanted in spring.
All types of beet will cross with one another, and since the flowers are wind pollinated, crossing can take place with any other flowering beet plants within around 2 miles. How fussy you need to be about crossing depends on what you are trying to achieve. If you simply want a reasonably diverse population of leaf beet, a degree of crossing is not that important. Plant your seed plants closely together in a square, and take seed from the central plants in the block; you will find that the amount of 'contamination' is minimal providing there aren't large numbers of other flowering beets right next door.
If you are aiming to keep a variety true to type you need to isolate it, usually by physically covering your seed plants. To do this, plant at least six plants very close together in a circle, with a wooden stake in the middle. As the seed stalks form, growing up to four feet tall, tie them together, supported by the stake. Then as they develop cover the group of flower heads with either a shiny paper bag that will withstand rain, or a bag made out of agricultural fleece. Shake the bag from time to time to make sure that pollen is distributed within the bag.
As the large, prickly seeds mature, keep an eye on them, and start to harvest as they turn brown and start to dry out. You can either cut entire seedstalks, or harvest mature seeds by rubbing them into a bucket. Make sure that the seeds are thoroughly dry before storage, and they should last at least five years.
Carrots are biennial, flowering in their second year of growth. In areas with mild winters, leave your carrots in the ground, mulching them heavily. The foliage will die back in autumn, but will then resprout and start to flower in the spring. In colder areas, dig up your carrots in the autumn, and select the best coloured and shaped roots. Twist off the foliage, and store the roots in a box of dry sand in a frost free place, making sure that they don't touch. In spring, replant the roots, and they will resprout and flower.
If you want to maintain a carrot variety effectively, you really need to save seed from at least 40 good roots to maintain good genetic diversity. If you have too small a genetic pool, you will end up with small, poor quality roots in a very few generations.
Carrots grow into big plants waist high or taller, producing successive branches with large flat umbels of flowers. They are insect pollinated, and need to be isolated from other flowering carrot varieties by at least 500m in an open field situation. This is not normally a big problem, since few people let their carrots go to seed. However, they will cross with wild carrot (Queen Anne's Lace), giving thin white useless roots. As with all insect pollinated crops, barriers such as houses, tall hedges and other high crops can affect insect flight paths drastically, so you don't necessarily need to eliminate all Queen Anne's Lace within a 1/2 km radius; but do watch out for any white roots in subsequent generations and get rid of them.
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 slightly more thickly to allow for the chaff mixed in. Carrot seed is relatively short lived, but if it is stored somewhere cool and dry, it should give good germination for 3 years.
Some common herbs
Basil, coriander and dill are annuals, parsley is a biennial, flowering in its second year of growth.
Basil flowers are insect pollinated, and different varieties flowering within around 150’ of one another may cross. On a garden scale, if you want to grow several types of basil, just keep picking the flower stalks off of all the varieties apart from the one that you want to grow for seed. Once several flower spikes have set and the flowers have started to wither, mark those spikes for saving seed from, and you can then allow the other varieties to flower. The seeds are ready to collect when the spikes turn brown and dry out. Don’t worry about the seeds dropping out – they are well attached, and actually need quite a lot of rubbing to free from the dead flower heads.
With both coriander & dill, to get the best seed for sowing in future years, pull up and discard the earliest plants to bolt, and only save seed from those plants that produce plenty of leaf and flower late. It is best to plan to save seed from early summer sowings, to allow plenty of time for the seed to mature and dry on the plant. Harvest as soon as the seed is brown and dry, as it does tend to drop from the seed heads. Rub the heads together in your hands over a bucket to free the seed. Dill seed usually comes cleanly away from the seed heads. Coriander seed tends to contain more chaff, but you can winnow it by pouring gently from one bucket to another in a light breeze if you want to clean it for kitchen use.
To save parsley seed, overwinter at least two or three plants. In warmer areas mulch heavily with straw or cover plants with a frame, elsewhere grow a few plants in a polytunnel or greenhouse. The next spring, the plants will start to flower and produce seed. Flat and curly leaved varieties will cross, as the flowers are insect pollinated, so you should only grow one type for seed at a time. Harvest the seeds from individual flowerheads as they dry and turn brown, as they tend to drop from the plant when ready.