Sweet Peas can be very successfully grown in containers providing a few basic rules are observed. These are;
Use the same timescale and method for sowing seeds as if the final planting was in the ground. If seeds are to be sown direct into a container this can be done from the end of March. A useful tip is to place a large clear plastic bag over the container whilst germination takes place. After seedlings have been “stopped” the bag can be removed and the support structure installed.
Ensure there are sufficient drainage holes in the base of the container and place a layer of coarse aggregate (not limestone) sufficient to cover the bottom, approx. 2-3cms deep. General purpose compost can be used as a growing medium but better results can be obtained by using a 50/50 mixture of good garden soil and general purpose compost, with 10% of the volume made up with well-rotted manure, at least 3 years old. In the absence of manure, a controlled release fertilizer can be used following manufacturer’s recommended dosage. Fill the container to within 5cms of the rim. Top this off with neat compost - this will act as a mulch and help retain moisture. It also assists when planting or sowing.
If netting is used as a support structure it will need to be draped from a higher structure or a wall, or fence. Often the larger containers have handles; the base of the net can be fastened to these. If growing against a trellis, some strings or net will be necessary to train the immature plants onto the trellis. The most common supports are canes of sufficient length determined by the type of Sweet Pea. These can be placed round the rim at 20cm intervals and tied together at the top to form a wigwam. Garden twine is then spiralled round the canes, tying it to the cane about every metre. This forms a stable structure that withstands wind and rain. However, the more vigorous growing varieties form a dense head at the top of the canes where it is difficult to tie them in. An alternative to avoid this problem is to erect the canes vertically with their tops attached to a wire or plastic ring, only slightly less in diameter than that of the container. Garden string is again wound round the canes to create a cylinder. This will contain the growth better than a wigwam.
As with growing Sweet peas in the garden, it is essential to remove the flowers as they appear to keep continuity of flowering.
Intermediate varieties need the same growing medium but with shorter supports. Dwarf varieties need no support at all and will grow very successfully in low troughs.