Cold weather has led to low numbers of pollinating insects, while the prolonged winter means plants are flowering up to a month later than usual.
On 7 January, wild narcissus flowered "exceptionally early" at three weeks before the average for the past decade. But since then, cold nights and days in February and March meant other bulbs have been three weeks late blooming.
Cold ground conditions meant Narcissus 'February Gold' came out in the first week in March, three weeks behind the average for the past decade.
At Kew, wildlife and environment recording coordinator Sandra Bell said: "Because of the cold most bees have gone back to bed so there's not any pollination."
Plantlife Cymru conservation manager Dr Trevor Dines said wild flowers are flowering up to a month behind last year. "There's no sign of wood anemones or bluebells and this time last year they'd been out for a long time. Anything that pollinates won't have the nectar around when they come out. The plants will lag behind the insects and that means the pollinators will struggle."
Buglife entomologist Steven Falk said: "Pollinators that do 'ecosystem services' are suffering badly because they need warmth and sunshine to access flowers. Mining bees and hoverflies are struggling to take advantage of spring blossoms affected by day length but many insects need temperatures of 12 degsC to be active."
He added: "We're worried about the use of pesticides, habitat fragmentation and loss of flowery habitat - and bad weather exacerbates all that." Falk said should the cold weather continue into April, then commercial crops of Prunus and Malus could be hit by low pollination rates.
Seasonal variation Late-flowering means lack of food for bees
Dr Kate Lewthwaite, who manages the Woodland Trust's Nature's Calendar project, said: "Spring is a long way behind a normal year - at least two or three weeks late."
She added that hawthorns and bluebells should be flowering but are only just forming buds. "This is going to be a real problem for insects because they only have limited fat reserves when they go into their winter sleep and are having to last a lot longer without food.
"If we lose queen bumble bees we lose pollinators and that has a potential knock-on effect for pollinating fruit crops. If bees come out when it gets warm they will not have much to feed on."
The trust has had just 22 sightings across Britain of horse chestnut trees in bud burst - this time last year it had 129 - and 300 spotters countrywide are yet to see a butterfly this year.