There are MANY factors to consider:

• The original research was done with fresh, wet-milled corn gluten (not corn gluten feed, corn meal, corn starch or corn meal)
• It did not work when a different area tried to reproduce it
• It had spotty results with commercial products that were scahing in on the initial buzz.

It appears to be rainfall sensitive - too much rain and it doesn't work. It also appears to be volatile or have a short shelf life.

http://extension.oregonstate.edu/gardening/corn-gluten-meal-did-not-prevent-weeds-germinating-osu-study

Try "staling" your vegetable beds ... get them ready, water them well and let the weeds sprout. Kill the weeds. Do it again.

https://lazygardens.blogspot.com/2016/11/dealing-with-weed-seeds-in-compost.html

I've used corn gluten in my garlic and onion beds for a couple of years. I put it down in the early spring when I plant my onion starts and the garlic has leafed out as well. It's not perfect, but given how onions and garlic hate weeds, it sure makes the weeding job easier. I have never tried it on a freshly seeded row.

1. If I do need to use it in place where I have direct seeded a crop, how big do the veggie plants need to be before it safe to use the corn gluten?

Varies from crop to crop. Broad leaf crops would be most easily affected. Might have some benefit on your wild violets issue

2. How long does it linger? Once I have used it on a particular area, when is it OK to plant seeds that I want to grow?

Not long at all. As already mentioned it is quite rain/water degradable.

3. Is it reasonably effective?

Only marginally so IME. Multiple applications, especially after any rain, can provide approx. 50% weed control in an area that has no other activity (such as a dormant garden bed).

4. Are there particular plants that are not affected by it?

Not affected at all? Rhizome weed varieties.. Only marginally affected - grasses and any narrow leaved weeds. It's primary effect is on broad leaf plants.

Not to stir up the till/no till debate but IMO repeated tilling is one of the most effective weed control methods. Till, water well, let weeds just barely sprout, till again, repeat as needed, etc. The thick use of cover crops and mulch over the winter can also be effective to a large degree.

Dave

1. The initial feeder roots at germination are what's affected. Once those spread further the plant is safe.

2. Five to six weeks is the generally accepted length of effectiveness.

3. I find it reasonable effective on my lawn. I have not used it in beds.

4. Any plant that has grown past that initial feeder root stage would be unaffected at the 10-20 lbs per 1000 sq. ft. application rate. A test where 80 lbs per 1000 sq. ft. was applied did damage to strawberry daughter plants in greenhouse trials. Lawns showed no damage at that rate.

Corn gluten meal works best if rained or watered in after application and then allowed to dry.

tj