My only dealings with bonds have been thru Treasury Direct but even with paper bonds the tax is due in the year they mature, even if not cashed.
https://www.spencerlawfirm.com/2010/06/income-taxation-of-savings-bonds/
from the US Treasury...apologies for the length. click the link to be taken to the relevant information.
https://www.treasurydirect.gov/indiv/research/indepth/ebonds/res_e_bonds_eetaxconsider.htm#when
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When must I report the interest on my tax form?
You have a choice. You can
report the interest every year
put off (defer) reporting the interest until you file a federal income tax return for the year in which the first of these events occurs:
you cash the bond and receive what the bond is worth, including the interest, or
you give up ownership of the bond and the bond is reissued, or
the bonds stops earning interest because it has reached final maturity
Reporting the interest all at once at the end
Most people defer reporting the interest, putting it off until they are filing a federal income tax return for the year in which they receive what the bond is worth including the interest.
When electronic EE Bonds in a TreasuryDirect account stop earning interest, they are automatically cashed and the interest earned is reported to the IRS.
You can see the interest on your IRS Form 1099-INT.
If a financial institution pays the bond, you will receive a paper 1099-INT from that financial institution either soon after you cash your bonds or within the first two months after the end of the year in which you cash your bonds.
If you cash electronic bonds in your TreasuryDirect account, your 1099-INT will be available early the next year in your account. (Video)
Reporting the interest every year
You may, however, choose to report the interest every year.
You may, for example, find it advantageous to report interest every year on savings bonds in a child's name. The child may be paying taxes at a lower rate than will be true years later when the bond matures.
Note: You (or the child if a bond is in the child's name) do not actually receive the interest every year even if you report it that way. The interest that the bond earns is reported on a 1099-INT after the bond is cashed or is reissued to reflect a taxable change in ownership. The 1099-INT will show all the interest the bond has earned over the years. Go to IRS Publication 550, Investment Income and Expenses, for instructions on how to tell the IRS that you already reported some or all of that interest in earlier years.
Once you start to report the interest every year (for example, for a child in the child's Social Security Number), you must continue to do so every year after that. for all your savings bonds (or, for example, all the child's bonds) and any you acquire (or, the child acquires) in the future.
Our online Savings Bond Calculator can help you determine the year-to-date earnings for the calendar year.
How do I report the interest?
Whether you are reporting interest at the end of the bond's life or every year, you report the interest from your bonds on your federal income tax return on the same line with other interest income.
If you are reporting the interest on bonds another person owns (for example, the interest on your child's bonds), you report that on the other person's federal income tax return with other interest income that person has earned.
Even if you’ve chosen to report interest every year, the 1099-INT that is reported after the bond is cashed will show all the interest earned from the date of issue. See instructions in IRS Publication 550 on how to report interest in this situation on your federal income tax return.