I am a landscape contractor and I hear this kind of thing all the time-- unfortunately the bad contractors give a bad rap for the rest. I agree with Mike F about having some plans drawn up before you get the contractors to build and before you even start collecting bids. This will make the bidding process for the contractors much easier, as they will have the dimensions and the materials laid out for them-- no guess work. The hardest part of my job is when I get to a potential new client's house, and they say to me. "Yes, I want an all new garden, but I don't know what I want. How much is it going to cost?" If you work with a designer in advance, a lot of the guess work is eliminated, and instead of saying, "It's going to cost anywhere from $10,000 to $30,000", the contractor will be able to tap down a logical cost.

The problem with hiring a garden designer or landscape architect to draw up the plans, is a lot of time they have never worked a day in the garden, so they are gleaning all of their plant info from books and what their teacher told them. Most times, the plans that I review have way too many plants in them as the designers don't take into mind the eventual growth size of the plants, or they are trying to give their client instant full-garden satisfaction. This mentality leads to a culling of crowded plants two to three years down the line, and a waste of money-- the original cost of the excessive plants, and then the cost of a gardener thinning them out plus hauling.

Another problem is the varieties of plants chosen. If the designer is just thumbing through a book, and adding plants that they think are pretty or matches the color scheme, they may not understand the viability of said plant. Only a gardener with a few years of hands on experience is going to know how a plant grows in your area, which pests and diseases it is prone to, if it drops a lot of litter, if it is high-maintenance. If you do hire a designer, then hire a different contractor, it would be best to be flexible with the contractor for the ultimate end game as far as the planting is concerned, but hold firm on the hardscape design.

Ideally, you could find a designer/builder to work with, that also has maintenance gardeners on their crew, or come from a maintenance background.

PS: if it is a patio with wide cracks, such as a dry-set flagstone patio, you could use a product called "Gator Dust" to tamp on top of the sand between the stones. After installation, give it a light sprinkle of water to moisten it, and it will take on a mortar like texture. It is fine decomposed granite mixed with mortar dust, and tends to last a couple of year before cracking, and will need refreshment then, but it will definitely keep the weeds at bay in the mean time.