Purslane – The Super Weed

Debby Luquette
Adams County Master Gardener

(8/15) It’s been a really bad summer for gardens in my corner of Adams County, even for the weeds. Except for one weed – purslane. Purslane is a succulent that defies heat, drought and dry wind.

Why is any weed worth talking about? Well, gardeners deal with weeds regularly. They can be frustrating, obnoxious, diabolical . . . pick your favorite adjective. What gives weeds these characteristics? Why are weeds so successful while we struggle to grow beautiful dahlias and luscious tomatoes?

More to the point, what is a weed? A weed can be considered a plant that is undesirable in a particular situation. According to that definition, a tomato that pops up in your flower bed is a just as much weed as crabgrass in your lawn. The plants we usually think of as weeds, though, are often troublesome because they have characteristics that allow them to thrive, out competing the plants we cultivate.

Why are weeds so successful? They generally have one or more traits that give them a competitive edge. In general, a weed needs a seed or reproductive piece, giving it access to a space, and usually, one or more tricks that allow it to compete with the plants you want in that space. One purslane plant can produce 240,000 seeds in a growing season, and those seeds can still germinate after 40 years! Do you think you can just break it up with a hoe? Forget it. Each piece of stem or root can become a new plant. Other weeds can produce offspring plants from stolons or rhizomes that extend out from a mother plant to increase their successful colonization of an area. Notable weeds using this strategy are Canada thistle and tree of heaven.

After a seed geminates, weeds have different options to achieve their goal. The basic strategy is to grab as many resources as it can, as fast as it can, so that it can successfully mature and produce seeds. We talked about one strategy already, producing as many offspring as possible to grab all the space it can. The other strategies involve monopolizing resources – primarily water, minerals and light.

To compete for light, the best thing a plant can do is to quickly push itself above the rest of the plants, either by growing tall, spreading out, or growing over the plants already occupying a space. Kudzu is noted for taking over a space by covering its competition, while giant ragweed takes the get-tall-fast route. These are not likely to be a problem for home gardeners since there generally isn’t enough room in a small garden for them to get started, but you see this strategy at work along the sides of roads. We’re more likely to encounter dandelions and crabgrass which have prostrate, or spreading, behavior. My problem weed, purslane, spreads 2 feet or more across.

When weeds compete for water and minerals, roots become important. Often the plants engaging in this strategy start with some roots, enough to get and nourishment, then put out leaves to gain energy in photosynthesis to feed the expanding roots.

Crabgrass forms a shallow, dense, fibrous root system that doesn’t look impressive, but it works. With lots of roots near the surface, crabgrass grabs scarce water before the deeper roots of my carrots and potatoes can get it. Dandelions and weeds that form taproots not only have access to deeper minerals and water, but that taproot anchors the plant, making it almost impossible to completely remove. If you fail to remove the entire root of a dandelion or pokeweed completely, it will be back, relying on the store of food in the remaining root to grow. Purslane doesn’t seem to be interested in a deep storage root, though it has a small but persistent taproot.

These differences in anatomy aren’t the only weapons in a weed’s arsenal. Many have physiological adaptations to make their survival more likely. We know that in photosynthesis a plant takes sunlight energy, carbon dioxide (CO2) from the air and water from the ground to produce ‘food.’ CO2 gets into the leaf or other green portions of the plant through pores that open when it’s light and close when it’s dark. Moisture inside the leaf is lost when the pores open for CO2, but under ideal conditions, this isn’t a problem since the roots are absorbing water.

This summer is different. When the pores open in hot weather, especially if it’s windy, water is lost faster than the plant replenishes it through the roots. When the soil is dry, it makes a plant’s life even harder. Some plants have variations in their mechanism for grabbing CO2 from the air, enabling the plant to carry out photosynthesis more efficiently. And, yes, many weeds have these tricks, too.

One variation allows the plant to open its pores are night while it’s cool and less water is lost. Cacti and most succulents carry out this variation, called Crassulacean Acid Metabolism, or CAM, by botanists. Cacti grow more slowly because this is not an extremely efficient method of photosynthesis, but it helps succulents deal with hot, dry conditions. The other common variation – called C-4 photosynthesis because of the way CO2 is handled chemically – is seen in many grass family plants, like corn and crabgrass. Generally, if a plant uses one of these alternative mechanisms, CAM or C-4 photosynthesis, it cannot use both. You guessed it. Purslane can do both, making it a superweed. It is a plant that nature designed to survive and thrive.

Interestingly, humans appear to have used purslane as a food plant early in history. It shows up in archeological sites in the 7th century and in lists of edible plants in the 4th century. A quick web search brings up a few modern recipes for this weed. So, how do I deal with this superweed? With my tomatoes and green beans suffering from the heat and drought, this adaptable nuisance may be the answer. Purslane quesadillas, anyone?

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