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This is when the Fed should hike rates: Siegel

The Federal Reserve should wait until the end of the summer to raise interest rates, rather than hiking in June as expected, Jeremy Siegel told CNBC on Tuesday.

"Everyone talks about how very strong the employment reports are, the payroll reports. What they're not talking about is how amazingly weak the GDP growth is in light of this strong employment," the Wharton School professor said in a "Squawk Box" interview.

The U.S. economy grew by 2.6 percent in the fourth quarter of 2014, the Commerce Department reported last month. Analysts had been expecting growth of 3 percent.

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"We just had terrible productivity growth. We economists are trying to explain it. That concerns me a little bit," Siegel said.

It would be premature for the Fed to raise rates in June when much of the world is heading toward zero or negative interest rates, Siegel said. He said the disparity in interest rates will send the U.S. dollar even higher, which is is equivalent to tightening credit on demand.

A strengthening dollar will make U.S. exports more expensive, possibly enough to offset the benefit of lower oil and gasoline prices, he said. Low energy prices suppress input costs for companies and boost consumer spending as Americans spend less at the pump and more elsewhere in the economy.

"[The Fed doesn't] have to add another increase on top of the dollar. I would very much prefer that looking at the situation, we don't see an increase until the end of the summer given the data that we're seeing right now," he said.

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