Growing British Native plants from seed is easy.
Many British Native species of pond plants like Caltha palustris, Alisma plantago aquatica, Iris pseudacorus, Rorippa nasturtium aquaticum and Ranunculus flammula will seed down into the mud or the water and produce small plants for next year if you leave them to their own devices. A sure fired method of gaining extra plants for your pond and bog garden.
They produce masses of seeds so you can collect some to allow you to sow seed in a greenhouse or cold frame to grow more plants in a controlled way.
But if you do not want extra plants then now is the time to remove the seedheads before they ripen and drop their seed naturally.
In the moist emergence zone Primula veris, Lychnis flos cuculi and Succissa pratensis will set seed too and spread their clumps in the moist soil. Tap the dried seedheads now and let the seed fall onto the soil and these new seedlings will come true.
Some non Native plants come true from seed - Iris versicolor, Cyperus eragrostis but Named varieties of pond plants and bog garden plants should not be allowed to set seed as seedlings will not produce the named variety but the parent plant - so remove these seedheads to avoid the plant wasting effort. These named varieties must be propagated by splitting the parent clump - vegetative propagation.