Sunday, January 19, 2014

What Works Well in Raised Beds (and What Doesn't) Part 1




I now have two years of experience growing vegetables in raised beds, so I am getting a handle on which vegetables grow very well in raised beds and which don’t. I am using the Square Foot Gardening method promoted by Mel Bartholomew. SFG uses a soilless medium called “Mel’s Mix”, consisting of one third peat moss, one third vermiculite, and one third blended compost. Replanting is done by adding additional compost to the squares being replanted. In Part 1 I will concentrate on the positive and talk about the vegetables I have had success growing in my raised bed garden. Part 2, to come later, will talk about the negatives.


The use of the soilless mix in raised beds has many advantages:

  • The soilless mix has excellent mechanical properties, being light and fluffy, well-aerated with good water retention.

  • Raised beds thaw and dry out quicker in the spring so planting can start earlier.

  • The square foot method provides very efficient use of space, maximizing production from a given area.

  • Weeding is easier and beds can be kept relatively weed-free.

  • The pH of the mix is near neutral so there’s no need for liming or soil tests. The blended compost (replenished every planting) provides organic matter, nutrients and most trace minerals.

  • Watering is easier and the beds make efficient use of water, retaining it and minimizing runoff.


So which vegetables work well with the SFG method? In assessing that question, you first have to factor out all the variables caused by weather, pests and disease. I experienced  plenty of those elements the last few years. In general, vegetables with reasonable root masses and reasonable nutrient requirements should grow well in raised beds. A square (foot) in a six inch raised bed will have a soil volume of only half a cubic foot for plants to grow in and supply nutrients. Of course roots are free to spread into neighboring squares, but if they do too much of that, production in those squares will be affected.


Leafy Vegetables

All of the leafy vegetables I have grown (lettuces, endive, escarole, chard, spinach, parsley, and even kale and collards) have grown very well. They like the light, well-drained soil mix in the beds. Weather is the biggest factor affecting them. since they tend to like cool weather. Since I plant them fairly densely, slugs can be a problem but are easily controlled with Sluggo. Flea beetles and cabbage moths are problems on kale and collards. Those can be controlled either with row cover or sprays approved for organic gardens like Spinosad and Pyrethrin.


Small Root Vegetables

The root vegetables I grow (beets, radishes, turnips, carrots) should be happy in the Mel’s Mix. My biggest problems with them have been weather and pests. I have had the most luck with beets, although they grow slowly and occasionally get Cercospora leaf spot. I am going to experiment with adding a boron (Boraxo) supplement this coming season. Apparently beets (and broccoli) respond well to boron in the soil (even Mel recommends the use of a boron supplement).


Pests are the biggest problem with the other root vegetables. Radishes and turnips were plagued by flea beetles, which have become a severe problem in my area the last few years. In addition, the cabbage root maggot and  carrot fly maggot cause a lot of damage. The cure for all that is row cover, which is a real nuisance to manage. I may have to consider consolidating all these crops in a single raised bed so I can cover the entire bed. That’s unfortunate because they lend themselves to being tucked in here and there in the garden to maximize the harvest.


I grew carrots in raised beds last year for the first time and they are a bit challenging. I used Granny’s seed mat planting method, but germination times were long and germination was spotty. The carrots were supposed to get a good start before the kale and collards achieved any size, but that didn’t happen. The other consideration for carrots is length. I looked for varieties that grew to a length of 5-6 inches. You can  use a “top hat” to extend the depth of the bed, but that is far more trouble than I am willing to take on. The carrots pulled earlier were beautiful, but the later carrots were riddled with carrot fly maggot damage. The only reason I am going to plant them again in 2014 is the superior flavor of fresh carrots.



In general, beans (both bush and pole) grow very well in raised beds. I have good crops every year with no problems. I usually use seed inoculant, particularly in a new bed, since you never know if the right kind of bacteria are present. Studies have shown that even in real soil, inoculants help increase yield. One thing I have noticed is that during the first few weeks of plant growth in spring, the leaves of the plants are a bit yellowish and become darker green as daytime temperatures increase. I attribute this to the fact that the nitrogen-fixing soil bacteria work better in warmer weather.



Peas also grow well in raised beds. I just grow snow peas and snap peas. Regular peas yield too little for the space they take. And I always use inoculant before planting. My past practice has been to select varieties that only grow 2-3 feet tall. I confine them in the bed with some short fencing and let them grab onto each other for support. The problem with this technique is it is a bit difficult to find the peas in the jungle of foliage to harvest them.



Cucumbers do well in raised beds but need to be trellised. Cucumber beetles (which spread bacterial wilt) and powdery mildew (PM) are the two biggest problems. It helps to pick resistant varieties. PM can be controlled with sprays, but the beetles are a real problem. I usually seed cucumbers directly in the soil, but last year the weather caused germination problems. I had to revert to starting seeds indoors in pots and then transplant them in the garden. If you can manage the diseases and pests, and the weather is not awful, cucumbers will do well in raised beds.


Onions, Garlic, Shallots

I have had good luck growing onions and garlic in raised beds. They are shallow rooted, like light soils but don’t like competition from weeds. The raised bed environment is ideal for them. Last year I planted onions in the row next to the tomatoes. They did well and so did the tomatoes, whose roots had grown into the neighboring row but under the onions. The only conflict occurred when the onions tops were falling over and I was supposed to stop watering them, but I knew the tomato roots were in the same area and needed watering.


Garlic from cloves planted last fall did well. I harvested the bulbs in July and was pleased with the yield. So much so that this year I planted five different varieties. My one failure was trying to grow shallots from bulbs planted in the fall. The heavy snowfall cover last winter caused the bulbs to rot and not a single one survived. In 2014 I am going to try growing shallots from seed, like onions.


Summer Squash

If you stick to bush-type summer squash and give them enough room, summer squash will thrive in a raised bed environment. They like the light soil rich in organic matter, so I make sure to add plenty of compost. The trick is to pick varieties that are fairly reasonable in their space demands yet will produce a reasonable amount of fruit. I plant them in a 3x3 square, usually pushing 4 seeds into a circle in the center of the square. For zucchini, Dunja is a great variety. It forms a compact, open bush which allows good air circulation and makes it easy to spot fruit before they become baseball bats. It pumps out beautiful dark green fruit and it is highly resistant to PM.


The path to success revolves around pest and disease management. Around here, squash bugs and squash vine borers are a plague. Add in PM later in the season and there is a lot to deal with. Row cover helps with  the pests, but when the squash start flowering, you are faced with the choice of hand pollinating or removing the cover. Planting PM-resistant varieties helps, as do sprays, for the PM.

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