My Treasury Ladder Cost Me a $1,180 State Tax Break — In the Best Account to Lose It

At 11:20 on a Thursday night in January, I was lying in bed with my phone six inches from my face, scrolling thru my brokerage app instead of sleeping, when I found the line item that ruined my week. Interest income, 2025: $14,382. Reported. Federal tax form 1099-INT, box 3. And underneath it, in the app's little tax summary: "State tax-exempt — verify with your state."

Wait. What?

I'd built a rolling ladder of Treasury bills through 2025 — four-week, eight-week, thirteen-week, rolling maturities into new auctions like a woman juggling knives she'd only read about. The interest looked fantastic compared to what my old savings account paid. What I hadn't reckoned about, not once, was what that interest did to my state return.

the exemption I didn't know I was using

Here was the part I'd slept thru: Treasury interest is exempt from state and local income tax. All of it. Every coupon, every discount, every bill that matured at par. It's written into federal law — states can't tax federal obligations. My $14,382 of Treasury interest had been flowing thru my taxable account all year, state-tax-free, while my state's top rate sat at 8.2%.

Run the math with me. $14,382 × 8.2% = $1,179.32. Eleven hundred and eighty dollars a year, every year I hold this ladder, that I am legally not paying to my state. I had accidentally executed one of the cleanest tax moves available to a resident of a high-tax state, and I found out about it at midnight from a footnote in my own brokerage app.

My first reaction wasn't joy. It was suspicion. When something in a brokerage app tells you the tax news is good, you check twice.

where the confusion started

The problem is that "interest" is a bucket, and my bucket had three different things in it. Treasury bill interest: state-exempt, no question. Interest from the money market fund my cash sweep uses: mostly state-exempt too, but only in proportion to the fund's actual Treasury holdings — the fund publishes a percentage each quarter, and you claim the exemption on that slice. Interest from a corporate bond fund I'd snagged in a moment of boredom in April: taxable everywhere, no exemption, no mercy.

My state return form didn't separate these for me. It asked for one number — interest income — and then offered a subtraction line for "US government obligations," with an instruction booklet three paragraphs long. I spent a Saturday morning with the instruction booklet, my 1099-DIV, my 1099-INT, and a fund factsheet, allocating my own interest like an accountant who resents her client.

The money market fund's percentage was 92% Treasuries for the year. The corporate fund's was zero. My final exempt figure came to $13,907. The other $475 of fund interest that wasn't Treasury-backed? Taxable. Fully.

the account type trap

Then I found the part that would have actually cost me money. The state exemption follows the asset, not the person — but only in taxable accounts. Inside an IRA or a 401k rollover account, the character of the income is irrelevant, because nothing is taxed til withdrawal anyway, and withdrawals come out as ordinary income regardless of what earned it. Hold Treasuries in an IRA and the state exemption buys you nothing. Hold them in a taxable account and it pays you back every single April.

I had mine in the right place by dumb luck. I'd opened the taxable account years ago for an index fund position I stopped adding to, and when rates rose in 2024, the ladder went there cuz that's where the idle cash lived. Right asset, right account, zero intention.

my neighbor, his rake, the wrong account

My neighbor did the opposite. Same ladder, same year, but his sits inside a rollover IRA he rolled from an old employer plan. Same Treasuries, same interest — and his state owes him nothing back. He found out from me, over a fence, holding a rake. The look on his face was worth the free advice.

the state-by-state mess

Not every state plays the same game, either in what it taxes or how it makes you prove the exemption. My state wanted the subtraction documented; my neighbor's state, two borders over, taxes municipal bond interest from other states while exempting its own, a rule I only absorbed because he complained about it with the rake still in his hand. The rules also varied wildly by state. Some have no income tax at all, so the exemption means nothing — you're exempt from a tax that doesn't exist. A few states tax municipal bond interest from other states but exempt their own. And at least one state I read about has its own special forms for fund-level pass-thru exemptions with worksheets that run four pages. If you move states mid-year with a Treasury ladder running, I truly don't know what your preparer's week looks like, but I hope they bill hourly.

The 1099 forms don't fix this either. Box 3 of the 1099-INT shows Treasury interest, but brokerage tax summaries vary wildly in how they break out fund-level Treasury percentages, and some don't at all — you hafta dig into each fund's year-end tax statement, published in February, and do the allocation yourself.

what changed in my setup

Three things. First, I stopped treating "interest" as one number — my spreadsheet now has a Treasury line, a fund line, and an everything-else line, updated quarterly. Second, I moved my emergency cash out of the corporate bond fund and into pure T-bills, cuz in an 8.2% state, a 4.3% corporate yield and a 4.0% Treasury yield are not actually close — the Treasury wins after state tax and it isn't suspenseful. Third, I set a February phone reminder labeled "fund Treasury percentages" so next year's allocation doesnt require archaeology.

I also did something I'd avoided for years: I actually calculated my after-tax yield. Treasury at 4.0%, state-exempt: 4.0% effective. Corporate at 4.4%, fully taxed at my combined marginal rate: closer to 3.1%. The "boring" asset was beating the interesting one by nearly a point, invisibly, all year.

the midnight checklist

If you hold Treasuries in a taxable account, know three numbers: your state's income tax rate, your funds' Treasury percentages, and the split of your own interest income. Multiply the first two by the third and you'll find either a refund you've been missing or a form you've been filling out wrong.

My $1,180 is staying exempt. The ladder keeps rolling. But I read footnotes now — at reasonable hours, with the phone on the dresser, where footnotes belong.