Around late October and early November, we start getting a lot of inquiries related to coriander, silverbeet and parsley. Usually the inquiry is from a puzzled gardener who has been growing great plants all winter, but suddenly everything has bolted (flowered), and is about to set seed.
The process of bolting occurs like clockwork. We manage many gardens across Melbourne and the silverbeet all set seed at the same time. Within a week of each other, all the silverbeet plants start flowering regardless of the individual microclimates. The same thing occurs with parsley, dill, fennel, coriander, spinach and rocket.
The process behind this phenomenon is called photoperiodism. This is a physical reaction and developmental plant response, to the relative lengths of daylight and darkness, that the plant experiences.
The Earth tilts on its axis throughout the year. This means that in winter, the days are much shorter, than in summer. Conversely, the nights are much longer in winter than in summer. Once we pass our Winter Solstice, the days start to increase in length until we reach the summer solstice.
Plants have evolved to use these lengthening days as a cue to flower and set seed. The cue for most leafy greens is increasing daylight hours, often over 12 hours.
For example, you may have a variety of coriander that bolts once the day length increases beyond 12 hours.
On September 19th – the day length will be 11 hours and 59 minutes
On September 20th – the day length increases by 2 minutes to 12 hours and 1 minute.
This triggers the coriander plant to start flowering.
The process of initiating flowering, will take weeks for the central stem and the flower to develop, so it may be a while before you notice it (which is why most of our inquiries about this don’t start until October).
The photoperiodism phenomenon is so hardwired into plants, that even tiny seedlings will bolt at this time of the year. It’s actually the length of the night rather than the length of the day that influences the plant. We know this because researchers have experimented with plants by increasing the day length and night length under controlled conditions.
It’s important to note that photoperiods i.e. the length of daylight (or more accurately the length of darkness required to induce flowering,) not only vary from species to species of plants but also between strains and varieties. So you may have one strain of coriander plant that bolts after 11 hours and 50m minutes of day length and another that bolts after 12 hours and 35 minutes. Often you can find varieties of leafy greens that are labelled as “slowbolt”. This variation is important for seed saving. If we continually select and save seed from the plants first to bolt, then we will start to select for quicker bolting varieties. It’s better to wait and select seed from individual plants that are slower to bolt, so that hopefully you can develop your own strain that is “slowbolt”.
Not all vegetable plants are influenced by photoperiodism. Some plants that don’t rely on the sun to induce flowering include corn, peas, tomatoes and cucumbers.