When the foliage is yellow/brown/dead. If it is still green, it feeds the bulb for the next year.

If your Tulips are not Darwin or species, they will not return next year anyway. In such case, you can dig the bulbs up and throw them away any time.

why wont they return? they have returned for the past 4 years. I'm pretty sure they are not darwin.

The vast majority of fancy tulips are annual.

Species are the best for returning year after year. Some Greigii, Triumph, Emperor perennialize too. Probably, you have some of these.

Clip off the flower heads and allow the green foliage to die back. This technique lets the plant put all its energy into building a strong bulb for next season. This is a common recomendation for all bulbous plants.

I always tie my leaves as they start dying off, it then gives you the room you want to plant other things.

tulips need to store the energy for next year..

if you mow off green.. no storage.. and eventual decline..

i would remove the dead brown leaves.. when they gently pull from the soil ... this is not a power tool deal ...

there is nothing to stop you from inter-planting annuals there now [without disturbing the bulbs below] ... while the leaves finish their work

ken

Tying up bulb foliage impedes its ability to photosynthesize and thereby re-energize the bulb for the following season. The recommended advice is to leave all bulb foliage in place (not tied up, not cut back, not braided, etc.) until it browns and dries on its own and comes away from the bulb easily.

From the MSU extension office: "Let's start by addressing the issue of braiding (or tying with rubber bands). It may seem like a tidy practice, but it is not in the bulb's best interest. Bound foliage is partially obstructed from sunlight, and is not able to photosynthesize at peak capacity, resulting in less energy going into the bulb for next year's show. This practice can also trap moisture and encourage foliar disease. Although it is not as tidy, it is best to leave ripening bulb foliage open to full sun and good air circulation, do not bind it or flatten it to the ground."

And while ripening bulb foliage is not very attractive in the garden, it is very necessary for the long term health of the bulbs. That's why many gardeners intersperse spring flowering bulbs with later-to-emerge, larger perennials, who's foliage can effectively 'hide' or disguise the appearance of the ripening bulbs.