Labour has pledged four new bank holidays on national saints days, three of which fall in the main gardening season.
The party also says it refuses to make "false promises" on immigration and believes in the "reasonable management of migration" but "will not make false promises on immigration numbers".
The holidays would be on each nation's patron saint day - St David's Day on 1 March, St Patrick's Day on 17 March, St George's Day on 23 April and St Andrew's Day on 30 November.
Labour leader Jeremy Corbyn believes the move would "celebrate the national cultures of our proud nations" as manifesto promises come out ahead of the election.
England and Wales have eight bank holidays a year, Scotland nine, and Northern Ireland 10. Labour says the average for G20 countries is 12.
Horticulture consultant Neville Stein said: "More bank holidays would be absolutely brilliant for garden centres. They would be welcomed by retailers, who are open anyway so have no extra costs but not by manufacturers, who will have to carry the costs without being open.
"It's a no-brainer for the garden centre industry. On bank holidays people will go to garden centres and enjoy a nice lunch or coffee and will buy gifts and will buy plants if it's the right time of year. Shopping is the biggest leisure activity in the UK on bank holidays."
Labour also says it will accept the EU referendum result and "build a close new relationship with the EU" prioritising jobs and and workers' rights, and guarantee the rights of EU nationals living in the UK and work to "secure reciprocal rights" for UK citizens elsewhere in the EU.
The party wants negotiating priorities to have "a strong emphasis on retaining the benefits of the single market and the customs union" and to negotiate transitional arrangements "to avoid a cliff-edge for the UK economy" if no deal is reached.
Labour has also pledged to keep EU-derived laws on workers' rights, equality, consumer rights and environmental protections, and to keep Sunday trading laws as they are, as well as banning neonicotinoids.
The Lbdemas have voweed to "suspend the use of neonicotinoids until proven that their use in agriculture does not harm bees or other pollinators".
HTA horticulture head Raoul Curtis-Machin said: "We have mixed feelings about the leaked manifesto. Any increase in bank holidays in the key planting season is to be welcomed, to give gardeners more time to indulge their favourite hobby. But we are disappointed that the Labour Party wants to maintain the status quo on Sunday trading. This is an out-dated law, much in need of bringing into the 21st century. We very much welcome the tree planting ambitions, but as we have seen before with previous governments, it is very easy to make manifesto promises without delivering on them.
"It is encouraging to see a reassurance for EU migrant and seasonal workers, and a new Agriculture and Horticulture Skills Council could be very welcome but we would not like to see a return to the old Agricultural Wages Board. It would be good to see a new sector skills council which helped with careers promotion, employment and skills data collection and analysis."
MDJ2 Consultants director Andy Newman said: "Labour's pledge to create an extra 4 bank holidays is probably nothing more than a 'crowd-pleaser' in truth. Whilst our existing bank holidays are massive trading peaks for garden centres, the new ones pledged are fixed dates which means they would often fall midweek.
On that basis it looks pretty unlikely that these would offer any significant upside for the industry. I would suggest that the policy issues that are more important to growth across the garden centre industry relate to disposable income and home ownership."
Stein said he agreed with banning zero hours contracts: "Workers need security, peace of mind and guaranteed incomes. Give them this and they might perform better. I understand though that companies might need temporary workers and need to switch on and off the supply of labour - those organisations such as fresh veg / fruit growers could have this problem solved by reinstating SAWS."