We started using cinderblocks/hollow concrete blocks years ago in creating small organic kitchen gardens. We did notice that the plants growing in and near the blocks were growing healthier than in the other beds. For our organic kitchen garden this was a great boost to the way we grow herbs and veggies.
For the small kitchen garden where space is everything we now recommend the use of these blocks as dividers for selected herbs and companion plants.
The blocks are also great for short term growing of scallions/spring onions, shallots and garlic. Apart from the obvious best use of space, there are other advantages to using cinder blocks in your small kitchen garden, including:
Using cinder blocks increases the pH of the plant. This is good for most small kitchen garden plants, especially for most organic herbs and vegetables who dont like acid soils
Cinder blocks are a great source of calcium for the plants you are growing in your organic kitchen garden
Porosity of the cinder blocks has its benefits such as:
a. Increased root growth. – Basically stops water logging of the root area and improves all round root aeration. The root hairs also love the rough porous surface inside the blocks and tend to really thrive there as it gives them a way to climb and attach to.
b. Improved drainage and water holding. Whilst cinder blocks are relatively porous and well drained, they also store water so they act as a reservoir for your small kitchen garden plants.
c. Better cooling and warmth. The evaporative cooling effect in summer can be significant in keeping the roots cooler is really noticeable when the blocks are used as a bed edge with soil or potting mixture inside. In the cooler months the moisture in the blocks assists in retaining heat (released at night). This effect is particularly noticeable in a greenhouse.
d. Increased access to other water and nutrients. Being bottomless has its advantages. In my small kitchen garden for example, the blocks are sitting on a bed of gravel (for good drainage). I have heavy duty black poythene sheeting under this gravel as a weed control measure. What happens is that the drained and excess water (& nutrients) are held by this plastic sheeting, acting as a water and nutrient reservoir. Also, over a period of time, spilt soil and mulch accumulates on top of the gravel, making it an even better reservoir of water and nutrients.
Cinderblocks are NOT really suited for larger plants or those plants requiring to be dug out at harvest (e.g. eggplant and potatoes). They are well suited to certain low growing perennial herbs and companion plants that otherwise would need space elsewhere. For example, chives, birds-eye chillies, nasturtiums, marigolds.
John Bee, Master Gardener explains that through using cinder blocks, the best way to grow veggies is to allow the blocks to separate your plantings and let the magic of porous cinder blocks seep water to the roots of your small kitchen garden