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I'm not sure what exactly you mean by 'the stable Rose Kennedy'. I am making the assumption that you mean a stable version of the converted tetraploid.
Breeding a treated 'Rose Kennedy' to itself will not produce a stable 'Rose Kennedy' it will produce a range of seedlings all genetically different from 'Rose Kennedy'.
When a diploid is treated with colchicine there are several possibilities of what happens.
1) Nothing changes. The treated diploid does not change; it remains a diploid with no tetraploid tissue.
2) Some part or parts of the treated diploid become tetraploid. There are many parts that can become tetraploid and there are many parts that may remain as diploid. I will break this down further below.
3) All parts of the treated diploid become tetraploid. This is a very unlikely result, but not impossible. If this does happen the treated plant is no longer diploid; it is tetraploid and stable.
Lets look at possibility 2 in more detail.
The part of a daylily that produces the above ground tissues is the growing point or shoot apical meristem.
There are three parts to the growing tip/point or shoot apical meristem of the daylily. These are basically three layers. Two of the layers (1 & 2) produce some of the male and female gametes, the other layer is not involved.
When the diploid is treated and some of the tissue becomes tetraploid one might have all of a layer turn tetraploid or all of two layers turn tetraploid. Or one could have just part of one layer turn tetraploid or parts of two layers turn tetraploid. In cases where less than the whole layer turns tetraploid and sometimes in cases where less than two whole layers turn tetraploid the conversion is not stable with time. Sooner or later the diploid tissue that was left might outgrow or outcompete the tetraploid tissue and the treated plant reverts to being all diploid.
A stable conversion may be present from the beginning (after treatment with colchicine) if an entire layer is converted or if two entire layers are converted. Although it is possible that with the passage of enough time, when only one layer is converted to tetraploid that some diploid tissue grows into that layer from the diploid layers and that the 'stable' conversion reverts to being a diploid.
Otherwise a conversion becomes stable when a layer that is only partially tetraploid manages to become completely tetraploid (with the passage of time).
The passage of time affects the identity of the growing point or meristem. When a fan blooms the growing point is consumed in the process of producing the scape and buds. A new growing point must take over the role of being the shoot apical meristem. This can happen because in the space between each daylily leaf and the crown there is the potential for a bud. These are called axillary buds (they are in the axil of the leaf with the stem - the crown of a daylily is a modified stem). So after the treated fan blooms there is a new growing point. By chance/luck when the axillary buds are formed they may have their layers be entirely one ploidy (tetraploid) even if the original treated meristem only had partially tetraploid layers.
The colchicine treated/converted 'Rose Kennedy' was either stable when it was converted or by chance, with the passage of time, and new growth, it became stable when partially converted layers (1 or 2 or both) of the meristem became entirely converted tissue.