What Makes a Weed a Weed? The Case of “Weedy” Rice

Over the past year and a half, I have been conducting research and extension with weedy (red) rice (Oryza sativa L.), which has re-emerged as a pest in California rice fields. What makes this plant so interesting is that it is the exact same species as cultivated rice (also Oryza sativa L.). This means that it is edible (fit for human consumption), not subject to quarantine (not regulated at the state or federal level), and difficult to distinguish from cultivated rice in the field, since it looks a lot like the cultivated rice varieties. It is, however, considered a pest, due to its impact on yield and processing quality.

Because it is rice, and it is edible, my colleagues and I have had to do a lot of thinking (and explaining) to growers and Pest Control Advisers as to why this particular type of rice is considered “weedy”, and why controlling it matters to growers. In explaining this in early 2016, it quickly became apparent that it would be important to define exactly what makes one rice plant a weed and another a crop.

This led to conversations about the early days of agriculture, and the desirable traits that humans selected for when domesticating crops. One of the most important traits that was selected for was the elimination of seed shattering for grain crops. Normally, an uncultivated (or “wild”) plant does not retain its seeds at maturity. Instead, they shatter (fall off the seed head). It is to a wild/uncultivated plant's advantage for seeds to shatter, so that the seeds are in the soil, ready for germination, when conditions are right. Like a wild/uncultivated plant, weedy rice populations tend to shatter off the panicle (seed head) more readily, rendering it difficult or impossible to harvest. Thus, weedy rice, though edible, does not make for a profitable crop.

The other critical trait that humans selected for was the elimination of seed dormancy. If a seed is dormant, it can remain viable in the soil for many years, without germinating, even if all conditions are perfect for germination. There are a set of dormancy-breaking conditions (particular to each species), that will then “prepare” the seed for germination. Many wild/uncultivated plants have seed dormancy. Cultivated/domesticated plants do not have seed dormancy, so that when a grower plants the seed in the ground, it will readily germinate, and go on to produce a crop. Weedy rice populations have varying degrees of seed dormancy. A grower that plants weedy rice seed will have low germination rates and thus, poor stand (low numbers of plants per acre).

In 2016, we started surveying our rice fields to identify weedy rice populations. However, we soon found that there were many potentially weedy or off-type rice plants present. We had to decide which set of characteristics made a population of rice “weedy”. We decided that populations having either seed dormancy or shattering, or both dormancy and shattering, were weedy. Other off-type populations, while perhaps undesirable, were not considered “weedy”. We used these two traits as a measure of weediness, because they negatively impact rice yields, and increase management difficulty.

So, how do we define a weed? In the case of weedy rice, it is by the lack of two key traits of domestication: retention of seeds until harvest, and high germination rates at planting.