The real problem with extending the seasons on Earth isn't that the plants can't cope per se; as has been mentioned by John in comments, plants on earth are already designed to react to their environment. When spring comes early, plants sense the change and start to bud. If winter is longer, then they just stay dormant that bit longer until they sense the changes in the environment that essentially 'activate them'. As such, you don't need any significant change to most plants to get them to support a longer cycle.
That said, a longer cycle isn't completely free of complications for your plants. Staying dormant during the winter cycle for longer increases the risk that the internal nutrients of the plant won't hold out long enough, and a more active cycle during the summer means that it's taking more nutrients and water out of the ground for a longer period and the soil and its nutrient and water base may not be able to support that for the extended period without some form of cropping - regular fertilisation and watering on the part of someone specifically there to care for the plants.
Of course, we're also assuming that the extended calendar is the only consideration here. Naturally, if Mars is significantly colder than Earth even after a substantial terraforming effort, then the plants might 'think' it's still winter, even in the depths of its summer period. While out of scope of the actual question, this is an important consideration as the plant will get about half the sunlight that it gets on earth at Mars, and if the temperature is still cold then it's a fallacy to think that the plant will react to a 'relative' cycle of hot and cold.
Ultimately though, there is a place on earth that doesn't really experience significant temperature variations, doesn't really have a summer and winter cycle and yet in which plants thrive;
Think about it; During both winter and summer, the equator is exactly the same angle from the sun. If anything, spring and autumn are going to be slightly hotter than both summer and winter because they are the point of transition, and therefore the equinox at noon is the one time when any point on the equator is actually in full sun. But, for all practical purposes, the differentiation between the two seasons we regularly think of as the most extreme is least in equatorial regions.
As such, there are a lot of plants in that area that are not deciduous. There's no point for a plant in that region adapting to a cold / hot cycle. I'd argue that if you're going to put plants on Mars, and you can keep them warm, the plants from this region are the first to try out.
Mars will have the same dynamics at its equator, meaning that the only variation you need to think about is in fact keeping the plants at a surviveable temperature. This would be the first place to start and then you can worry about the planetary seasonal cycle after you have plants surviving on Mars without it. Don't unnecessarily complicate your terraforming efforts as there are far more important considerations to think about in getting plants to grow there than seasons. So take them out of the equation (no pun intended) first and focus your plantings on Mars' equator.
Doing that will in point of fact will serve as a great control for the experiment of getting plants to grow and take a variable out of the mix that could otherwise muddy the waters.