You can plant and grow directly into the hay or straw bale spacing the plants as you would in a garden.
Planting. Make a slit in the top of the hay with a spade and simply plant the plants you have selected. Small seedlings might benefit from a sprinkling of fine soil to give bit of support.
Sowing seeds. If you are sowing seeds into the hay or straw bale, its a good idea to sprinkle some compost or topsoil into the surface or slits you have made with the spade, this stops the finer seeds falling to deep into the bale.
Follow the instructions on the seed packet for depth of sowing if the slit is to deep to get the recommended
sowing depth top up the slit with soil, sow the seeds and cover with fine soil.
Sowing potatoes. Potatoes (all root crops like carrots, parsnips and swede) do well in bales. Make a slit with a spade and plant the potato in the center of the bale about an inch deeper than you would in soil.
Tall plants like some varieties of tomatoes will need support by pushing long stakes all the way through the straw bale to make them secure.
Also the very good compost that's left as the straw decomposes can be spread over poor soil to improve it. If you keep placing the new bales on top of the old ones you will eventually end up with some very good soil full of earthworms. If you keep this up, you could be on your way to the "no digging method of gardening" described on my website.
Almost anything you can grow in a garden will grow well in your hay bale garden, from cabbages, dwarf beans and many more vegetables and fruits from strawberries to tomatoes and can be planted as seedlings or sown as seed. The exceptions (unless you can support them) are tall plants like runner-beans.
You can make raised beds using bales in two ways;
The plants in straw bales are reachable from a wheelchair and a straw bale garden is relatively weed-free, quite handy if bending is difficult.
More information about raised beds in my article raised beds
Bales of straw or bales of hay can be sited almost anywhere in your garden, providing it gets a fair share of sunshine (most plants especially fruit wont thrive in shade) and water, even on rocky soil, hard heavy clay soil, or concrete and patio slabs.
Hay bale or straw bail fruit and vegetable beds are perfect for small paved gardens, where planting and growing space is limited.
Can I position bales in a soggy part of my garden ?
If you have a boggy or poorly drained area in your garden you can build the hay bale over the area, you will need to water less frequently, or maybe not at all if its really waterlogged, as the hay or straw bales will soak the water up into the bale.