Raised Beds: Perfect for Your Herbs and Vegetables
Raised bed gardening in northern California is an excellent way to grow vegetables and herbs. The interest in building raised beds over the last few years has increased greatly. How do raised beds differ from container gardening? While gardening in containers is useful because it is more portable, with an enclosed bottom and ability to easily move, container gardening is open at the bottom, increasing the ability for drainage and allowing for huge yields.
Why build a raised bed? The most important reason is it will immediately enhance your soil. If you have heavy clay or less than ideal conditions, it could take years to properly amend the soil. With a raised bed you can immediately change the soil, making a nutrient rich, well drained soil ready for planting (almost) immediately.
A second reason is that raised beds are excellent for people who are physically challenged, raising the soil up to the correct height for wheel chair, cane or other aid. A lip can even be added, allowing gardeners to sit directly on the edge of the garden bed.
Raised beds can also be useful to exclude pests. Strong fine wire can be attached to the bottom of the box if gophers or moles are a problem, copper flashing can be attached to the sides of the bed to exclude snails and slugs, and it is easier to deal with children and small animals wandering through the garden and flattening plants.
Raised beds also warm up more quickly in the spring, and it is easy to attach covering to either keep the garden warm or exclude birds and insects.
What is the best size for a raised bed? The great thing about a raised bed is it can be built to fit in any location, so you can achieve best space efficiency, which is terrific for small spaces. In general, the width of a raised bed should be no more than 3 ft if you can access it from only one side, or 4 ft if you access it from both sides. The length of the bed can be as long as is comfortable for your space, and depends on what you are using to build the sides. For the height of the raised bed, at least 1 ft is probably needed if you are planning on growing larger vegetables and herbs. A two foot deep raised bed is common to avoid leaning over to garden or if you are in a wheel chair. It also gives you deeper soil that is well drained.
The most common material to use to build a raised bed is wood. Traditionally cedar or redwood is used, although price of these materials has increased greatly. Untreated fir can also be used, just remember that fir will not last as long. There are plant based stains and specifically available for raised beds that are safe to use with food crops that will extend the life of wood. Never use treated wood for beds intended for food crops. There are pre made wood raised beds available, with corners that are tongue in groove, allowing the wood to slide easily into the corners. Metal corner kits can be purchased separately so you can choose the type of wood, height and length for your bed. The important thing to consider with taller larger beds is that the weight of the soil can require that you add supplemental bracing along the length to avoid bowing of the structure.
Besides wood, raised beds can be made of stone, metal, or block. Corten steel is a very stylish choice that will naturally patina, while there are kits for metal raised beds that will give a farm look to your garden.
Once you have your bed constructed, you will want to fill it with soil. Think of the soil in your raised bed as the foundation of your plantings – don’t skimp on this first building block! Most of the money you spend will be between the construction of the bed and the soil. Never use native soil, as it will not drain well as part of a raised bed. Also, there are many recommendations to use rocks, sticks, cardboard or other fillers to make it cheaper to fill the raised bed. Try not to ‘stratify’ the soil. Drainage will be much better if you concentrate on providing a homogenous, well drained, nutritious soil mix throughout the bed. Layers of rock, gravel or sand just affect how the drainage will occur throughout the bed, providing uneven watering to the plants.
Many bulk soil blends are acceptable for raised beds, except they all tend to lack enough organic material to give good drainage. For bulk soil blends (purchased by the yard) plan to add an additional amount of compost or other good organic material. For a yard of soil (27 cubic ft) I would suggest adding 25% more organic material. It is also a good idea to add some earthworm castings or well aged manure to your bed.
So how much soil do you need? I love to use the pre mixed Raised Bed mix from EB Stone. In the wood 2 ft tall, 2 ft wide and 6 ft long raised beds we built last spring at the nursery, I needed approximately 24 cubic ft of soil. That would be 16 bags of the 1.5 cu raised bed mix to fill it to the top. I chose to use 15 bags of raised bed mix and 1 bag of earthworm castings that I used to mix in the final top layer for a little extra manure. If I was buying bulk soil, I would have purchased a yard of topsoil (27 cu ft), and added 2 bales (6 cu ft) of Big Harvest compost and 2 bags of 1 cu ft earthworm castings to enrich the soil. Since it is difficult to have less than 1 yard of bulk soil delivered, you will end up with a little extra that can be used for another project.
Its always a surprise how much soil you need to fill raised beds. Once they are filled however, keeping the soil healthy is very simple. If you are planting annual crops you will want to add a bag or two of replacement soil and some earthworm castings. If you plant both summer and winter crops this means adding soil twice a year. If the bed rests in the winter, then placing straw, alfalfa pellets or planting a cover crop like fava beans will add some nitrogen and organic matter back into the soil. Fertilizing, especially with slow release organic fertilizers will help maintain healthier soil.
Rotating crops is also critical to keeping the soil healthy, especially if you are growing tomatoes, peppers and eggplants – all members of the nightshade family. It is a good idea to have 2 raised beds – one for tomatoes, peppers and eggplants, and one for squash, beans and cucumbers, and switch them every year. This prevents the build up of diseases in the soil that are specific to the nightshades, especially tomatoes. If you can’t rotate your crops, be sure to grow peas or fava beans in your bed in the winter, and turn in a few new bags of soil into your bed before planting in the spring to help maintain a healthy soil.
If you have paid attention to your soil, watering a raised bed will be fairly straight forward. You should have a soil that is both well drained and will hold water and nutrients. The idea of a raised bed is that you shouldn’t need to water a raised bed every day, it should be less work, lower impact and use less resources. Try not to water very lightly every day, but to water deeply. Installing drip lines, soaker hoses or spray heads is a good idea – you can even have them set up to screw on to hose ends so the watering system can be seasonal. Hand watering will also work, but it is better to water slowly, near the roots, allowing water to soak into the soil slowly rather than spraying the plants. Water enough to soak deeply – at least 1 ft.
Raised beds are perfect for herb gardening, making it easier to harvest and enjoy the plants we love! Sunny raised beds are perfect for the Mediterranean herbs like sage, thyme, rosemary, savory and oregano. Plant those herbs on the driest, hottest side of the bed, and keep the herbs that need more water and fertilizer – basil, parsley, fennel, chamomile - together in a separate area of the bed. Plant the large growing herbs like lemon verbena in the middle and use those plants to protect the shadier loving herbs like cilantro and chervil. Keep the mints out of the raised bed – best to put them in their own pots since they will be too happy in the beautiful soil you have created in the raised bed. Plants like turmeric and ginger would love a shadier raised bed where they could thrive all summer and would be easy to dig up and harvest in the fall. And for those of us in gopher country, raised beds with hardware cloth on the bottom are the only way to grow garlic all the way to big beautiful heads.