Mosgiel rose grower John van Delft said overnight temperatures as low as 6degC in Dunedin recently had stalled the growth of his roses.
To meet an order for 4000 roses - ``nearly all red'' - for distribution across Otago today, he cranked the coal and diesel boilers to heat the 0.4ha glasshouse.
The roses were given more water than usual to boost growth.
``We got there in the end. It came good in the last few days.''
He had been growing roses in the nursery for nine years and the demand for them on Valentine's Day had remained steady, he said.
Roses were being imported from India and Colombia to meet the demand of the Valentine's Day market, he said.
The colder New Zealand temperatures produced a rose with a bigger flower and a stronger stem, he said.
Nicol's Garden Centre Dunedin florist Marie Driver said red roses were the most popular flower choice on Valentine's Day but this year a mix of white roses and white Oriental lilies was ``quite trendy''.
When Valentine's Day occurred mid-week, more people gave flowers, she said.
On a week day, people sent flowers to workplaces to embarrass the recipient.
When other staff saw the gift, it prompted them to buy flowers, resulting in busier florists.
New Zealand Law Society president Kathryn Beck warned sending an anonymous card or roses to a colleague today could be construed by the recipient as ``sexual harassment or stalking''.
The ``anonymous admiration'' could come as an ``unwelcome surprise'' in the workplace.
If it was ``unrequited love'' and there was ``repeated and unwanted'' contact it could constitute sexual harassment and lead to ``serious legal problems for misconduct'', Ms Beck said.
