In order to get a continued succession of vegetables you need to plan it. Some will grow pretty much through the same sequence whatever the weather – others are very susceptible. We tell you which are which – and how to measure and manage the conditions.
Continuous supply of vegetables.
Problems of irregular supply can be put down to unpredictability of the weather. Seeds may germinate sooner or later than expected, the soil may cap preventing the seeds breaking through. Once they’re up the weather they experience may be variable. However with a bit of intelligent action the variability caused by temperature, sunshine and rainfall can be reduced.
Together these affect growth rate, yield and time to maturity while temperature can influence the development process itself.
Plants grow exponentially – like compound interest, the more they’ve already got in the bank the faster they grow. Plants in their vegetative phase grow quickly on a percentage basis growing at about 10% per day (radishes). Its also been found that plant growth doubles for every 10 degrees C. So if you put the 2 together , a 10g radish will weigh 35 g after a week
This is like compound interest – the more you have to start with the faster the growth
So if you use seedlings raised in a warmer temperature you get on quicker – that’s the advantage we have as commercial growers using a temperature controlled propagation area. However on a garden scale, even a moderate sized greenhouse can make a big difference to extending the season.
There is a certain minimum temperature that varies from crop to crop above which seeds germinate and plants develop normally so that sooner or later they will produce the root, shoot, leaf, flower , fruit, pod or seed that we are looking for.
What we have to recognise is that the best conditions are different at different stages of the plant’s growth. If we get the wrong conditions at critical points in the plant’s development we can lose production.
For instance, Butterhead lettuce seeds won’t germinate if the temperature is above 25 degrees. This means that in summer planned successional planting may fail. Once they have germinated, however, the higher the temperature the faster the growth, with the rate doubling every 10 degrees C. So in the height of summer you may be better to start them off in modules or seed trays in the shade or othewise making sure that they don’t go above 25 C until the seedlings are up.
Many plants need specific conditions to trigger the growth to the next phase – such as flowering and fruiting. In some plants we want to encourage this while in others we want to keep them vegetative. For example, cauliflowers need enough time below 21 C to trigger curd formation.
In the 70s, the research laboratory at Wellesbourne bred a number of varieties that need different lengths of cold to stop leaf production and produce curds – however the plants need to be big enough first if a decent crop is to be secured. Snowball for instance needs very little encouragement while some winter varieties need 12 weeks below 15 C before they start to curd. The prevailing temperatures will clearly tip the balance towards earlier or later crops. This is why a soil and a maximum/minimum thermometer are a key part of the Intelligent Gardener’s Tool Kit.
Controlling these effects relies on 5 techniques
- Use of average dates for sowing and planting
- Taking into account prevailing temperatures in successional sowings
- Sowing different varieties at the same time to guarantee succession
- Modifying the environment round the plants
- Developing new ways of growing crops.
Use of average dates for sowing and planting
For some vegetables you can go by the calendar for although the conditions may vary considerably from year to year, the timing of the harvest won’t vary much. Carrots, beetroot and parsnip may take longer to grow but they won’t deteriorate in the ground so sowings may be made at fairly wide intervals. Cabbages can sit in the ground for a couple of months and runner beans may be harvested continuously.
Other crops like onions from bulbs ripen predictably because their behaviour is controlled by the day length. With all of these there are no great problems in maintaining supplies over long periods of time.
Crops like lettuce on the other hand which have short harvest periods tend to need sowings on frequent, fixed succession dates. Intervals need to be longer in spring than in summer as the average daily temperature rises and falls the time to produce hearted lettuce will shorten and then lengthen again – as shown here
From Volume 2 of Bleasdale and Salter’s Know and Grow Vegetables.
Sadly out of print but can be sourced via Amazon.
The Authors publish similar graphs for cauliflowers.
Taking into account prevailing temperatures in successional sowings
Peas and Beans need successive plantings – but the right interval depends on the temperature. This is because since growth rates correlate with the ambient temperature – and this can vary dramatically from year to year.
One approach is to track the actual temperature profile since the last sowing using degree days.
You do this by measuring how far the average temperature goes above 5.6 C each day for most crops (although for some crops like French Beans and Sweet Corn the threshold is 10 C and for Cucumbers 14C)
Growth achieved by the plant depends on the number of degree days accumulated over time so you need to measure the daily total of degrees per day and maintain a running total. This means you need to get the average temperature for each day, subtract the threshold temperature and maintain a tally. Once the right number is passed you should plant the next batch.
If you take the readings with a maximum and minimum thermometer at 9AM each day, you can add the minimum and maximum temperatures together and divide by 2. Subtract the threshold temperature and that gives you the number of degree days accumulated for the previous day.
Tracking the running total will let you see how much growth has occurred so you can start the next sowing at the right time.
We can apply this to peas, French beans, sweet corn, tomatoes, lettuce, carrot and mini cauliflower. If you track this over a couple of years you will know how the plants behave in your specific location. When the right number of degree days has been accumulated since the last sowing you can sow the next lot. This will give you a reasonable succession irrespective of what the weather’s doing.
Use different varieties.
If a number of varieties that take different times to mature are taken of say Brussels Sprouts, French beans, cauliflower and onion are sown on a single date over a number of years, the varieties will always mature in the same relative order, despite variable weather crops. These are documented in NIAB leaflets but these are (were) slanted more to commercial varieties – as many as 6 varieties might be needed to cover the situation fully
Modify the plant environment
The main problem is low temperatures in Spring and Autumn reducing plant growth. Cloches and growing in protected environments is the main approach. This might get lettuce ready to cut up to 3-4 weeks earlier. Another approach is to mulch to warm the soil. This could be straw or biodegradable plastic. Best control is obtained with starting them off in a green house. The best approach is to raise them in modules (what we nursery types call seed trays but with little compartments for each plant)
Other cultural methods
You may try overwintering broadbeans, cauliflower, salad and bulb onions. They have to be big enough by late autumn but – as in the case with onions – not too big
Parsley and Parsnip can be pre germinated in doors and then fluid sown in March can give 2-3 weeks advantage. ~Brassicas can be grown and then transplanted and removal of apical dominance by pinching out Brussels, beans and tomatoes can help. New varieties of Japanese onions and non hearting lettuce have made early production a reality.
Finally control of size by spacing can make a big difference to the time of availability of crops like cauliflower. More information available about crop spacing here.