That’s odd: you planted a hybrid tea rose with big double red flowers and the next year, up came a rose with tiny single white flowers. What happened? Was it a mutation? A miracle? A sneaky neighbor switching plants? Actually, none of the above!
This is a very common situation with grafted roses. What happened was that the upper part, that is, the grafted rose with its big beautiful blooms, died during the winter, but its rootstock, a very different rose, survived and sent up shoots of its own. The resulting roses are small-flowered roses: Rosa canina (single pink flowers), R. multiflora (tiny single white flowers) or R. ‘Dr. Huey’ (semi-double dark red flowers). If you’re very observant, you can even detect the substitution before the new plant flowers: rootstock roses will have thinner, less upright stems than the original, plus the leaves will also be smaller and composed of more leaflets.
This unfortunate transformation happens in all climates, but more often in cold climates because the very popular bush roses, such as hybrid teas, grandifloras and floribundas, are not very hardy (about zone 7). So even if you give them appropriate winter protection (mounding the base of the plant with soil and installing a rose cone or a geotextile covering), there is usually still mortality. During a very cold winter, over half of these tender roses may die.
Why are roses grafted? Because, by inserting the bud of a good garden rose onto the stem of a fast-growing, vigorous but less attractive variety (the rootstock), the grower obtains a salable plant faster. A cutting-grown rose may be hardier and longer-lived, but it takes a year longer to produce.
So there is actually a very easy solution to this problem. To prevent unwanted sprouting from the rootstock, just avoid planting grafted roses! The latter are easy to recognize because they bear be a rather obvious swelling at their base (the bud union), that is, where the desired rose was grafted onto the smaller-flowered rootstock. If you’re having trouble seeing the difference, ask the nurseryman to point out some non-grafted roses.
Shrub roses, which are the hardiest roses (often to zone 3 or 4), are rarely grafted, but instead are usually grown on their own roots. They’re the best choices for cold climates. But if you find a good rose nursery, you can usually find a few own-root bush roses (hybrid teas, grandifloras and floribundas). They are worth seeking out in all climates, because if the upper part dies for any reason (and cold winters are only one reason a rose bush could die), the plant can often regrow from its roots. And since the rose is not grafted, it’s the desired variety that will grow back.
You can only find grafted plants of a favorite rose? In that case, it’s at least worthwhile planting extra deep so that the bud union (the swelling at the base of the plant) is buried 2 inches (5 cm) deep. This used to be suggested only for cold climate gardening, but now most rosarians recommend burying the bud union even in mild climates,
Why bury the bud union so deep? So that the stem of the desired rose is in contact with the soil. It is therefore likely to start producing its own roots from the buried portion of its stem and will eventually produce enough roots that it no longer needs the rootstock which will then slowly die. If so, and the top part dies back for any reason, what grows from the base will be the desired rose variety.