Virus induced gene silencing (VIGS) is definitely an option.
The ease of using VIGS depends on a lot of factors. Current VIGS vectors have limited host ranges. Depending on the species of plant that you are working with, there may or may not be suitable, pre-made VIGS vectors available and published techniques for using them. Many of the VIGS vectors that are available may be limited in terms of which tissues they can infect. Working with the vectors themselves requires that you have some way to propagate them and some way of getting them into your target plants (for example Biolistics, Agrobacterium or direct application of infectious virus).
There have been a number of VIGS vectors developed over the years, capable of infecting a diversity of plant species. Modified Tobacco mosaic virus (TMV) and Tobacco rattle virus (TRV) are two common VIGS vectors used for silencing in a variety of plants. TRV in particular is known to have a very wide host range. If your species doesn't fall within or even near the documented host range of available vectors it may be worth just trying a few different ones. Or, you could always isolate a virus specific to your target plant and build you're own. What fun!
The length of time that silencing will last is difficult to predict. In my experience, it will typically last a few weeks. But this may vary depending on a number of host factors and the VIGS vector used.
One potential alternative would be to just use Agrobacterium. It could be combined with a recombinase system to satisfy your condition of transience. To build such a system you could insert you're silencing construct into a binary vector. By including an inducible recombinase sequence adjacent to your silencing construct sequence and by flanking both of these with the recombinases recognition sequences, you would be able to remove inserted DNA at some point down the road after transforming the plant.
Again, depending on which species and variety of plant you're working on, this could require some trial and error as there may not be published techniques for Agro mediated transformation of you're target plant.
While Agro mediated transformation may not quite be what you're looking for in terms of transient silencing, it may be sufficient for a proof of concept.