What decade did the Spanish population IN AMERICA grow so rapidly?

The Spanish were the first European Americans after the Norsemen. Spain owned Florida, California, Nevada, Texas, Arizona and Utah.

So, if by "America" you meant the USA, check when the USA acquired those territories.

Hispanic and Latino Americans (along with Asian Americans, most notably) have contributed to an important demographic change in the United States since the 1960s whereby minority groups now compose one-third of the population. Nearly one in six Americans was Hispanic or Latino as of 2009, a total of 48.4 million out of the estimated 307 million Americans. High rates of immigration and fertility have shaped the growth of the Hispanic and Latino population.

If you mean: Spanish-speakers in the U.S., the trend is actually downward

https://www.pewresearch.org/hispanic/2015/05/12/english-proficiency-on-the-rise-among-latinos/?utm_source=Pew+Research+Center&utm_campaign=a46e44efae-&utm_medium=email&utm_term=0_3e953b9b70-a46e44efae-400022289

If you mean: Spanish people in the Americas, we have centuries, not a decade, of colonization.

The Obama debacle.