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My broad bean plants are near end of their life. I planted them in autumn, and now we are half way through our first summer month. Many have been blown over by strong spring winds. The bean pods are hard but most are still green, and still a ways from being black.

Near mature broad bean

Now, the advice online seems to be that we must let the beans fully mature in the pod on the plant, and we can't save seed from opening these pods and letting them dry indoors.

Same bean opened

What exactly is the issue? Do they go mouldy or are they just not mature enough? I'd really like to remove them now, and plant something else!

Update: a month later, I've decided to start harvesting some of the beans for seed. The pods are mostly black though not all are dry.

Beans ready

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Once the foliage dies back, the beans are mature, but must be dried. If you can't or don't want to dry them on the stalk, pull or cut the stems and hang them by the bases in a dry, airy place. Once the pods are completely dry, and the beans are hard, you can save them for storage.

If the foliage is still healthy and green, leave the pods on the plants. Yours look just a bit green yet. But if you cut them now, you'd probably still get a decent germination rate if dried and stored properly.

As long as the beans' 'umbilical cord' is still alive, it will be transferring nutrients/energy from the stems into the seeds. And they should dry in complete darkness (inside the pod makes that easy). Also, you are likely to damage the skin on the seeds if they are shelled before completely hardened.

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The pod is considered a specialised leaf structure, and in the Fabaceae family are formed from a single carpel, whereas in Brassica species it is formed from two fused carpels. Its role is to protect and feed the developing seeds and is connected to them by the funiculus as seen in the second image.

Initially, the developing seed (bean) obtains its storage material from leaves with the pod acting as a sink for the photosynthetic output of the plants vegetative material. But as the pod matures, the pod becomes a highly photosynthetic structure and then provides the source for the majority of the photosynthates stored in the seed.

With reference to the question asked, the pod undergoes senescence first losing its photosynthetic ability changing from green to black, and becoming thinner. This is thought to allow more of the light received by the pod to be transmitted to the seed to allow its own photosynthetic machinery to complete maturation. Some carbon dioxide is trapped inside the pod and can be used by the seed for this process. So, the answer is that the seeds should be left in the pod on the plant so that maturation can be completed.

https://www.ncbi.nlm.nih.gov/pubmed/21507003

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