This last summer has seen a number of ZB talkback listeners complaining about scarlet runner beans mis-behaving. Two main types of troubles were reported:

1) suddenly no more bean pods were growing despite the plant getting to its normal height.

2) At the start of harvesting mature beans, the leaves begin to turn yellow – going down and down.

Scarlet Runner Beans originate from Central America and they like well-drained soil (loam) in a sunny spot. 

Plant the seeds under a tall support to allow the plants grow up and up, the root systems will survive the winter for next year’s crop. They are perennials!!

These beans are Nitrogen Fixers. They don’t need much Nitrogen (they make their own), but love Potash and Phosphate. 

They are long-Season growers with red flowers, pollinated by bees and bumblebees.

Harvest the mature pods regularly and watering regularly to prevent them from drying out (not toooo wet, please!)

What could cause the sudden troubles with the formation of bean pods?

• Lack of Pollination (not enough pollinators);
• Hot weather, especially when that happens at night – lack of water would make that worse.
• Bees sometimes “rob” the nectar from flowers by chewing them open.
• Leaving pods on the plant too long may reduce future seed production.

I decided to contact my old friend and DSIR colleague from way back: Dr Keith Hammett, a well-known plant breeder and famous for creating amazing new varieties of sweet peas and… 

Dahlias (also a perennial plant!)

He came up with some great stories and a possible solution to the yellowing and ill-thrift of Scarlet runners: In the UK these climbing beans were grown as annuals: plant new seeds every spring. If you’d leave the tubers in the soil they were more than likely killed by frost. (“as would Dahlias” said Keith)

In New Zealand’s warmer climate zones we can keep the tubers in the soil, so they will be able to successfully over-winter (they behave like Perennials). Interestingly, the perennial scarlet runners produce beans a little earlier than the sown scarlet runners.

The comparison of Beans with Dahlias is quite interesting: We can leave Dahlia tubers in the soil for several growing seasons, but the performance of the older, perennial plants falls away. It is the build-up of previous year’s growth that tends to choke the plants.

Top Dahlia exhibitors would only ever grow plants grown from cuttings; “Youthful tissue and all that… with no old stuff in the way”

Keith has three tripods of Scarlet Runners (third season) on the go in his garden. One of them is showing those debilitating features with yellowing leaves….

He attributes that to stress and an old tuber giving up the ghost

For what it’s worth… maybe a good idea to sow new seeds in spring?