What makes plants bolt

Bolting is when a plant goes to seed prematurely. There are several possible reasons – water stress, day length, cold are all possible culprits. If we understand what the plant is trying to do we can manage our vegetable succession better. Because once flowering is triggered, it stops root swelling and makes stems woody – reducing the value of crops like radish and beetroot.

As gardeners, our aim is to maximise the yield of the bit of the plant that we want – and this may not suit the plant’s intentions at all. A crop like Pak Choy will bolt as soon as you look at it and the llfe of a hearted lettuce in the ground is usually less than a week.  That’s why we have to be intelligent in our approach to keeping the plants in the condition we want. We need to understand how nature works and look at the situation from the plant’s point of view if we are going to get the results that we want.

What a plant wants when it’s growing is an uncompeted supply of nutrients, water, light and air at a suitable temperature.   However their behaviour is subject to outside influences. Two of the most important are the temperature and the length of the day.

As the plant grows it goes through different stages  in which it behaves in specific ways. These are triggered by external factors and mediated via plant hormones – cytokinin, giberellic acid, IAA (auxin) and abscisic acid. Release of these can be triggered by external events such as drought or cold – so the cold of autumn causes the release of abscisic acid which triggers leaf fall by causing a band of cells at the base of the leaf stem to become corky and die. Once a plant has moved on from one stage to another there is no going back – even if it may take some time to happen.

For instance, bolting in cabbages is contolled by how many days have passed below a certain threshold temperature. Once the 6 weeks that’s needed has passed, flowering is inevitable – even if it takes another 6 weeks to happen.

Monitoring degree days using a max / min thermometer is one way of keeping track of how close you are to the edge with a particular crop.  J K Bleasdale in  his 2 wonderful books know and grow vegetables  describes how keeping good records in this way can really help manage succession of crops like lettuce.  Given that lettuce seeds won’t germinate in temperatures greater than 75F temperature monitoring is likely to become more important in the UK as climate change takes more of a hold.

This of course applies at optimum spacing – if they’re crowded together you get competition effects  either for light or for nutrients and yields are reduced

Widely spaced leafy crops that are harvested early in their life cycle maintain the highest relative rates of growth for longest.  Lettuce is very temperature sensitive.

So we can see that vegetable gardening is about removing as many factors that check growth as we can.

Growth is channelled in different organs at different stages in the plants growth – we can get more productivity by controlling it.

If plant’s get stressed they miss stages and bolt prematurely – it’s a real problem for the new gardener to understand what’s going on particularly with crops like Pak Choy or other chinese vegetables.

forinstance Lets talk about  onions for instance.

Initially low temperature holds back germination and growth. As the soil warms growth speeds up but the day length gets longer.

In the UK a daylength of 16 hours (by mid may) stops the production of ordinary leaves and induces production of the fleshy leaves that make up the bulk. Once this is started there is no way back. So to get maximum yield you need the best growth you can before this happens. You can try warming the seedlings earlier to get good growth before daylength kicks in or by using onion sets.

In Britain seeds are sown in 3rd week of May – only a month before maximum day length so you can sow thickly to get small bulbs. You can get enough sets for one allotment from 2 seedboxes.

Dormancy gets broken easily by wet. Its maintained above 70 degrees F and below 0 C – dormancy breaks at 50 degrees F.

If we plant large bulbs they flower but we don’t want this – we want them to grow with big reserves.

The low temperature of winter start a developmental responses in larger bulbs but smaller bulbs are “pre-pubescent” and are immune. So don’t use sets larger than 15mm in diameter unless heat treated by storing at 25-30 C to prevent them flowering.

We can also sow onions in the autumn using traditional long day varieties and then transplant them in the spring. These will start to bulb once we get to 16 hour days. Japanese onions by contrast can start to bulb with a 12 hour day at the end of March.

Bulbs bigger than 15mm will flower after exposing to low temperatures but smaller bulbs will not respond to the temperature and will produce big bulbs the following year.

If you grow under glass even with a January sowing you can protect them from the low temperatures that trigger flowering. The danger with planting too late on the other hand is that they may not survive the winter.

Traditionally we sow last week in August in South of England, Mid August in the midlands and first week of August in Scotland and North of England. Autumn sowing gives us a long period for varieties that bulb in short days and will give a yield in early June – they’ll be green and won’t store though.

Autumn sown onions bolt less if kept supplied by nitrogen fertilisers during the winter. Leeks can be grown from pips which are achieved by shaving off the flower heads before they openl

So Applying this to other vegetables

What causes Bolting?

Many vegetables are biennials – they’re not affected by cold as small plants but then need a cold stimulus to start flower production.

Some are winter annuals in the sense that as soon as the seed takes up water for germination they are susceptible to cold initiated flower production. Turnips are a good example.  Celery is transitional in that young plants are not affected as much as older plants.

The best vernalisation temperature is 4 degrees C.

Above 12-14 degrees there is no stimulus to flower and if the change happens quickly there is no stimulus to flower – a warm day will cancel out a cold night as we clock up the stimulus to flower.  If the days are cold as well then it starts to count towards the degree day tally.  6 weeks is enough usually to trigger bolting but you may not see the effect for several weeks later – January and February days will trigger May Bolting

TurnipVulnerableDelay sowing or protect crops
BeetrootSensitiveYoung plants susceptible – use varieties like Boltardy or Avon Early
RadishSensitiveUse variety suitable for time of year
CarrotsAutumn sowings a problem – vernalisation will occur if  1 true leaf is set
ParsnipsRelatively bolt proof
Celery/CeleriacVery SensitiveBecome more so as they get bigger. Most vulnerable at transplant size. Don’t slow them down by hardening – trim them instead at 3 inches high to get sturdier plants
Cabbage and SproutsHave a clear cut puberty like onions when the stem is pencil sized. Summer and winter cabbage are generally ok but spring cabbage is always on a knife edge. 13th July for northern sowings, 20 July for midlands, 27th for the south.
Chinese CabbagePushed to bolt by low temp <50F and long days. Don’t plant before mid may for transplants or midsummer for open ground
LettuceGoes from head to bold in 5-6 days. Winter glass lettuce may bolt without hearting
SpinachVery Long Day sensitiveOnly grown in Autumn, winter and spring

Vegetables we WANT to flower

CauliflowerVery variableAny check leads to buttoning. Transplant no later than 5-6 weeks post sowing cold treatment of seedlings makes them most uniform don’t transplant when hot and sunny
Runner beansPerennialsLift roots in autumn store them in frost free environment and plant out in early spring for an early crop. As long as you keep picking them, the plants will keep producing. Keep the roots moist so they don’t get checked.
Peas and BeansThey stop setting pods when their capacity is reached. At a pea spacing of 60 plants per sq m you will get 3-5 pods per plant – ie 300 pods per sq m. French beans behave similarly. At wider spacings you will get the same yield but flowering will carry on for longer.
TomatoesAlways flower but sometimes the first truss aborts because the plant is growing fast.  They may also fail to set due to low temperature. (5 C is when it stops – 25 C is optimum0
Cucurbits and MarrowsMake sure you don’t get male flowers on cucumbers – then they will be bitter

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And if you want to know how this works in practice visit our Fletching Glasshouses Organic Vegetable Site

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