"CO2 is plant food" is a fairly common skeptic argument (this answer is adapted from the SkepticalScience article on that topic). The basic point is that additional CO2 is good for plants, provided they are in an environment where CO2 is a limiting factor in growth, i.e. they already have adequate water, sunlight, nutrients etc. This is why additional CO2 is sometimes introduced into greenhouses, where watering, sunlight and feeding are already well optimized. However outside greenhouses, CO2 is not generally a limiting factor. Quite often the low availability of water (e.g. due to drought) or poor soils (due to over-farming) is what prevents productivity, and adding extra CO2 is of little or no use to plants. CO2 however, is a greenhouse gas, and can be expected to produce warming, which may have a negative effect on water availability, and for some plants temperature is a limit on where they can grow. Simplistic arguments are generally unlikely to be true on this one as there are many factors that are involved.
As an example where Dr Idso's argument clearly is not the case, consider this study:
Insects Take A Bigger Bite Out Of Plants In A Higher Carbon Dioxide
Date: March 25, 2008
Source: University of Illinois at Urbana-Champaign
Summary: Atmospheric carbon dioxide levels are rising at an alarming
rate, and new research indicates that soybean plant defenses go down
as carbon dioxide goes up. Elevated carbon dioxide impairs a key
component of the plant's defenses against leaf-eating insects,
according to a new article.
There are references to other studies at the SkS article mentioned above (especially the advanced version).
The problem is that you can always cherry pick studies to support a point of view, but the best course of action depends on taking a broader view of the scientific literature, and consider what is likely to happen in the real-world biosphere (i.e. you need to consider the local limiting factors around the world on a family-by-family basis). The IPCC WG2 report is probably a good place to start. I'm much more familiar with WG1 issues, rather than WG2, but this article suggests that the IPCC do not agree with Dr Idso.
I should however point out that Dr Idso is presenting this work at a conference for climate skeptics only, funded by the Heartland Institute (a political lobbying organisation). It isn't the sort of place he is likely to get a serious scientific peer review of his work that will point out the errors.