If you haven't already eliminated the thistles in your target area, it's probably time to switch strategies. The thistle plants are bolting - sending up straight, tall stems from the center of their rosettes (ground-bound circle of leaves), and thorny buds are forming at the top of the bolted stem. These are the flower buds.
<--- bolting purple starthistle plant ---
On the Italian thistle, some of these buds are starting to open and reveal the numerous purple flower petals (top photo). I will be mentioning Italian thistles often in this post because they are the first thistles to bolt and are currently flowering at the Dipper Ranch (2200-foot elevation) in our region.
Under Thistle Logic, I described how to tell the different thistles by the shape and markings on their rosette leaves. Now you can also sort the thistles by their flowering characteristics. Italian thistles have "decurrent" stems - the spiny edges of each pair of opposite leaves continue down each side of the stem like long fins or spiny ruffles. The Italian thistle flower buds are clustered together at the tip of the stem and they usually are 1/2 to 7/8" wide including the spines , noticeably narrower than other thistles common in the Santa Cruz Mountains.
Timing: Once an annual plant starts to bolt, it is determined to flower and produce seed. By timing your mowing correctly, you can use this inclination to your advantage. Changes in the chemicals circulating in the plant at the time of bolting cause the growing tip in the center of the rosette to produce elongating cells. Once the stem reaches approximately knee height in the Italian thistle, the growing tip switches to producing flower buds. Two to five flower buds form at each stem tip. These quickly swell into their spiny threat. The phyllaries (overlapping leaves covering the flower bud) open and the purple petals reveal themselves, although the flower heads on the Italian thistle never open into a broad face.
<--- decurrent stems of the Italian thistle ---
Thistles are pollinated by insects. You might see honeybees, native bees, flies, beetles and ants visiting the thistle flowers and thus spreading the pollen from flower to flower to create fertilized seeds. The insects may also be there to eat the seeds or lay their eggs among the thistle seed larder.
Each individual Italian thistle flower head is actually made up of about 14 small flowers bunched together. That means that each Italian thistle plant can produce hundreds of seeds. All that thistle bounty is from one original seed, so it's important to stop the flowers from developing viable seed.
<--- honeybee on milk thistle ---
Mowing down the thistle stems will stop the development of the flowers at their tips. Wait to mow until the plant has made a serious investment in stem growth - when the stem is about 1/2 its ultimate height or up until the time when the flower buds start to swell . If you wait until the flower buds have opened, some of the flowers may have already become fertilized; even after being severed from the nourishment of the live plant, there may be enough energy left in the stem and bud for the seed to continue to mature and become viable.
Repeat Mowing: The thistle plant, however, is still determined to flower and seed, so it will probably send up another or several more bolting stems in the next few weeks. Keep an eye on the resprouting and mow the thistles again when the stems are several inches high yet the flower buds are not open. Depending on your regional climate, the wet/warm periods of the current season, and the species of thistle, you may have to mow one to four more times in the summer.
<--- Bolting Italian thistles. This stand has no open flowers and is ready to mow. ---
Sometimes, plants will react to mowing by sending up very short stems and the new flowers will be so low to the ground, you won't be able to mow them with typical mowing equipment. Then you are at risk of letting the plants go to seed. This is a bad situation and the best way to avoid it is to make sure you don't mow the plant too early - make sure the plant has already sent up several inches of bolting stem. Each time you chop off a considerable bulk of the plant in the stem, you are removing much of its resources. Repeated mowing should eventually deplete the plant of energy. In fertile soils and with mild, rainy summers, the thistles may keep resprouting until the weather and day length finally trigger the end their annual lives.
In poor soils, or dry conditions, the thistles may grow to a shorter stature and produce smaller flowers sooner. The bolting of the stem, and formation and swelling of the flower buds will still follow a predictable pattern that you can track to determine the right time to mow.
--- Weevils mating on the leaves of an Italian thistle at the Dipper Ranch. These may be thistle head weevils (Rhinocyllus concicus), an introduced biocontrol insect that unfortunately may also attack native thistles. --->
Individual plants within a stand of thistles can grow at different rates. Try to time your mowing to get most of the flower heads at the right stage or err a little bit on the early side so fewer flowers are likely to go to seed. You can walk through a thistle stand before mowing and pull out the plants with opened flowers or cut off the ripe flowerheads. Put these into a container for disposal. To avoid throwing more material into the landfill, I usually stockpile mature/pulled thistle plants in the corral where the cattle eat and stomp on them. Even though some may continue to develop to mature seeds, if they germinate next year, they are likely to get beaten by the cattle again, or at least I can attack them all at once in one spot.
In conclusion, carefully time mowing of thistles and expect to repeat it several times in the summer. Soon, I will describe techniques to avoid harming wildlife while mowing, sheet mulching and how to tell if a seed is ripe.
Italian thistle, Carduus pycnocephalus
Purple starthistle, Centaurea calcitrapa
Milk thistle, Silybum marianum
- Weeds of California and Other Western States, Joseph M. Ditomaso, Evelyn A. Healy, 2007, University of California, Division of Agriculture and Natural Resources.