What was Jerome's defense for translating the Hebrew word קִיקָיוֹן (kikayon) in Jonah 4:6, etc. into Latin as hedera? In the Vulgate translation of Jonah 4:6–7, 4:9–10, Jerome translated the Hebrew word קִיקָיוֹן  (kikayon) into Latin as hedera, which is the equivalent of the English word "ivy."

Some, including Augustine, expressed their discontent with this particular translation of Jerome. In response, what was Jerome's personal defense for his translation of the Hebrew word קִיקָיוֹן into Latin as hedera?
This is a famous spat between Jerome and Augustine. You may have seen "The Very Secret Diary of St Augustine" that has been circulating recently:


  Correspondence Jerome continues. Infuriating. Do not understand why he does not see my point! Translation of "gourd" vital to understanding of gospels.


The argument plays out in a series of their letters. The basic point is the divergence between the Hebrew, Greek (both the Septuagint and other versions), and traditional Latin. The Septuagint called it a "gourd" (κολοκύνθῃ) but the alternative translation from Hebrew by Aquila of Sinope uses "ivy" (κισσός). These correspond in Latin to cucurbita and hedera respectively. The "Vetus Latina" traditional Latin versions followed the cucurbita route; the controversy arose when Jerome picked hedera instead. In the lengthy and snippy correspondence between him and Augustine, this was one of the points of debate.

Augustine wrote to Jerome (his letter 71 of 403AD) with concern that the bishop of Oea (now Tripoli) had been confronted by an angry congregation, dismayed at the "very different rendering from that which had been of old familiar to the senses and memory of all the worshippers, and had been chanted for so many generations in the church." He was concerned not just about the specific gourd issue, but with the reliability of the Biblical texts in general, and the danger of public confusion. Jerome replied (letter 75 in the 
Augustinian corpus, 404AD):


  I have already given a sufficient answer to this in my commentary on Jonah. At present, I deem it enough to say that in that passage, where the Septuagint has gourd, and Aquila and the others have rendered the word ivy (κίσσος), the Hebrew manuscript has ciceion, which is in the Syriac tongue, as now spoken, ciceia. It is a kind of shrub having large leaves like a vine, and when planted it quickly springs up to the size of a small tree, standing upright by its own stem, without requiring any support of canes or poles, as both gourds and ivy do. If, therefore, in translating word for word, I had put the word ciceia, no one would know what it meant; if I had used the word gourd, I would have said what is not found in the Hebrew. I therefore put down ivy, that I might not differ from all other translators.


In the commentary that Jerome mentions, he had written:


  For gourd or ivy in Hebrew we read qiqaion, which is also written qiqaia in the Syriac and Punic languages. It is a type of shrub or sapling with wide leaves like a vine, and which casts a large shadow and is supported by a trunk and often is found growing in Palestine especially in sandy areas. It is interesting to note that if the seed is cast on the ground it germinates quickly and in a few days it can be seen to have grown from a seedling to a bush. For my part when I was translating the prophets I wanted to just transliterate the Hebrew word seeing that Latin has no word for this kind of tree. But I feared that the men of letters would find in this some argument, imagining those animals of India or the mountains of Boeotia or even other marvels of this type. I have also followed the example of the former translators who translated it as ivy, in Greek chissos, because they had no other word to use.


Basically, then, Jerome wanted to pick some familiar word rather than an exotic/mysterious one, and felt that "ivy" was a better fit than "gourd" for this plant which is a bit like both, but not really much like either. In particular, his commentary points out that the function of the plant in the story is to provide shade, which is more naturally associated with ivy than with a gourd. He goes on to say that rejecting the bad arguments of the "gourd-lovers" shouldn't lead us to ignore the imagery of Isaiah 1:8, where Zion is compared to a hut in a cucumber field.

This did not quite satisfy Augustine, who replied (letter 82 of 405AD):


  My only reason for objecting to the public reading of your translation from the Hebrew in our churches was, lest, bringing forward anything which was, as it were, new and opposed to the authority of the Septuagint version, we should trouble by serious cause of offense the flocks of Christ, whose ears and hearts have become accustomed to listen to that version to which the seal of approbation was given by the apostles themselves. Wherefore, as to that shrub in the book of Jonah, if in the Hebrew it is neither gourd nor ivy, but something else which stands erect, supported by its own stem without other props, I would prefer to call it gourd in all our Latin versions; for I do not think that the Seventy would have rendered it thus at random, had they not known that the plant was something like a gourd.