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I was considering the creation a long-lived plant which generates harmful (to other species) radiation. If it could be immune to its own radiation, the primary benefit would be that the plants offspring can spread easier and more quickly due to other plants and animals in the area being sickened.
How could a plant generate harmful (only to other species) radiation, or a similar effect?
There are 2 things to consider while answering this question: How to stop the radiation, and how to make the radiation.
There are 3 ways to "stop" radiation:
1) Time - Not an option, because... well it just doesn't make sense. Typically, time is considered a method to "stop" or lower radiation because the radioactive materials decay - that would mean our plant is dead.
2) Distance - Not an option, because if we put the plant brings the radiation with it.
3) Shielding - This is our option. It's possible for our plant to use a form of biological shielding. By growing extra "skin" around itself (fairly thick skin), it would be protected by radiation by the absorbent layer outside. This layer can be shed and regrown as needed such that the radioactive layer is never saturated.
The basis of your plant at this point: Thick stalks, thick leaves, thick everything.
But that still leaves us with how it produces radiation.
It just so happens that all things produce radiation, even humans. Living things require potassium, and in every 8,550 potassium atoms is one radioactive potassium-40 atom. Lets use Bananas for a second - they contain high amounts of potassium, and about 600~ bananas contains enough potassium to emit 1 chest x-ray of radiation. In a world where the plants are able to concentrate these nutrients (perhaps some radioactive handwavium is in the ground that it absorbs and moves) into certain "flowers" or "fruits", you now have radioactive appendages that poison the surrounding area, while the plant itself is protected from radiation via its thick "skin".
Your plant now possibly looks like a really thick, green glowing flower with fruits hanging from it's stalks. Perhaps it has really really big disposable flower petals that are radioactive, and drop off as it grows more, such that it can increase the radioactivity of the area surrounding it quickly when the wind carries the petals away and scatters it into the wind.
SF writer Larry Niven postulated the "Slaver Sunflower", which use very shiny flower petals to focus sunlight on an energy absorbing organ where the Pistil and Stamen of Earthly plants are. This plant would have evolved on a world with very little or dim sunlight to need to focus solar energy like that, but (how is unclear in Niven's stories) the plants also evolved the ability to track the sun, sense the presence of other plants or creatures nearby and react by swinging around and focusing their "mirrors" on the offending creatures.
Plants caught in the beam of a sunflower would be burned and turned to fertilizer, while animals might be burnt, blinded or otherwise injured. If they died of they injuries on the spot, then they fertilized the ground for the benefit of future generations of sunflowers, otherwise their behaviour was modified to "stay away from the shiny plant". As sunflowers tend to grow in fields, the end result is the concentrated energy of several hundred plants gets focused on targets. Archimedes reputedly needed several hundred polished bronze shields to do something similar to Roman ships...
According to Niven, the end result of the release of Slaver Sunflowers in the wild was to create an ever expanding monoculture of sunflower fields as they burned away their competition. What happened when they filled an ecosystem was never clearly explained. Since the Slavers were a spacefaring race, they brought sunflowers and other invasive species with them wherever they went, so human explorers a billion years later were apt to get some nasty surprises.
The fairly obvious countermeasures would be that creatures evolved to eat sunflowers at night, or spray sunflowers with toxic chemicals to burn or wither the mirror petals or opaque "ink" to cover the reflective surfaces when illuminated by day. Approaching the sunflowers from the north so the plant cannot reflect sunlight on you is another ploy that creatures might evolve. Some creatures and plants might arrange some sort of symbiotic relationship to live within the sunflower fields, and sunflowers would probably fall prey to various blights and plant diseases (although the sunflowers themselves might have a means of detecting infected plants and burn them down out of self preservation.) Since Slaver Sunflowers are an invasive species, the natural predators and symbionts from the home world are absent, hence the long term threat that sunflowers present to an ecosystem.
Plants in general are not very vulnerable to radiation, there was no massive plant death in Chernobyl, for example, so being immune to it isn't really an issue. Conversely it wouldn't help it against other plants.
As for generating it, almost all everything organic has some radioactive components, so that plant just has to filter it out and concentrate it.
I'm not entirely sure about the radiation angle, but plants already protect themselves via chemical means. The Black Walnut emits juglone, which interferes with the development of plants withing the nightshade family. Nicotine and Pyrethrins discourage insects from eating the plants. Think about how toxic hemlock can be. These are all examples of plants being pretty aggressive in protecting themselves.
As per Wikipedia:
In 1970, about 160 Bikini islanders returned to live on the atoll after they were reassured that it was safe. They remained for about 10 years until scientists found an 11-fold increase in the caesium-137 body burdens and determined that the island wasn't safe after all. The 178 residents were evacuated in September 1978 once again.
What happened is that radiation levels had dropped to nominally "safe" levels, but only because the air and surface soils had been tested by the US Government. They failed to test groundwater, where the cesium-138 had concentrated.
The coconut trees and repatriated islanders' taro crop subsisted on this water, and cesium is chemically similar to Potassium, a nutrient plants absorb from soil. So became concentrated in the food the islanders ate, and eventually the islanders.
So I think all your plants need is a source of radioactive contamination in the soil and Nature will do the rest.
